Understanding VoLTE SIM: Requirements, Pros/Cons, and Best Practices

What Is VoLTE? 

Voice over LTE (VoLTE) uses 4G LTE networks to transmit voice calls as data packets. Unlike traditional circuit-switched voice calls in older 2G and 3G networks, VoLTE operates over the IP-based LTE data network. This allows simultaneous voice and high-speed data services, enabling users to make phone calls and use the internet at the same time without sacrificing connection quality. 

VoLTE offers improved call quality with high-definition (HD) voice codecs that transmit a wider range of audio frequencies. Additionally, VoLTE achieves faster call setup times and uses network resources more efficiently. As operators phase out older networks, VoLTE has become the standard for voice communication on LTE devices worldwide.

The Role of Subscriber Identity Module (SIM) in VoLTE

To enable VoLTE functionality, the device SIM card must support identity and authentication features tailored for IP-based voice services. Traditional SIMs used in 2G and 3G networks are insufficient for VoLTE. Instead, VoLTE relies on more advanced SIM variants, particularly the IP Multimedia Services Identity Module, which is a logical application on the universal integrated circuit card (UICC).

The SIM stores specific credentials used by the IMS (IP multimedia subsystem), which is the core system managing VoLTE call sessions. These credentials include the IP Multimedia Private Identity (IMPI) and the IP Multimedia Public Identity (IMPU). The IMPI uniquely identifies the subscriber within the IMS domain and is used for authentication. The IMPU acts like the user’s phone number in the IMS and is used for routing SIP-based calls.

In addition to storing these identities, the device must be VoLTE-compliant and capable of running SIP-based applications for initiating and receiving calls. The SIP user agent may act as both a client (UAC) and a server (UAS), facilitating complete two-way SIP communication.

Device compatibility is also critical. VoLTE functionality requires detailed handset certification and testing to ensure full interoperability with the IMS network. This ensures that SIP signaling, codec support, and identity management are implemented correctly, allowing the network to deliver consistent call quality and service reliability.

Understanding VoLTE Billing and VoLTE Roaming

VoLTE billing differs from traditional voice billing because voice calls are transmitted as data over IP networks. However, operators typically treat VoLTE calls as standard voice usage for billing purposes. This means that although VoLTE technically consumes data, voice minutes are still counted against a user’s voice plan rather than their data allowance. To implement this, mobile operators use policy control and charging functions (PCC) within the IMS and EPC (Evolved Packet Core) to distinguish VoLTE traffic from general data traffic and apply appropriate billing rules.

Billing also relies on accurate session identification using Diameter signaling protocols. The IMS core communicates charging events through the online charging system (OCS) for prepaid users or the offline charging system (OFCS) for postpaid users. These systems generate charging data records (CDRs) for each VoLTE call, including metadata such as call duration, QoS level, and IP bearer information.

VoLTE roaming introduces additional complexity. For seamless roaming, both the visited and home networks must support IMS roaming interfaces. This includes support for the S8 Home Routed (S8HR) model, where signaling and media traffic are routed back to the home network, or the Local Breakout (LBO) model, where media is handled locally and signaling is routed to the home IMS. The S8HR model is more common because it allows operators to centralize IMS services without requiring full VoLTE interconnects between operators.

IMS roaming also requires that both operators support compatible QoS profiles, SIP signaling formats, and codec configurations. Additionally, roaming agreements must define how to handle billing, including inter-operator settlements and the proper generation of roaming CDRs to prevent revenue leakage. VoLTE roaming is still limited globally, and many networks fall back to circuit-switched voice or rely on OTT applications when full VoLTE roaming is not supported.

Requirements for VoLTE Device and SIM

Here are the main technical requirements for a VoLTE device with a SIM card, which go beyond what is needed for 2G or 3G voice services: 

• Provisioned to work with the IP multimedia subsystem (IMS): This enables signaling, authentication, and call session control in VoLTE. This requires the SIM to store IMS-specific credentials such as the IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU), which are used for authenticating and addressing the subscriber within the IMS network.
• Supports session initiation protocol (SIP) registration and communication: This is essential because VoLTE relies on SIP for initiating and managing call sessions over the LTE network. SIP capability ensures the device can register with the IMS network and exchange signaling messages for call setup and teardown.
• Supports quality of service (QoS) parameters: The SIM and device must be able to signal and enforce QoS settings that prioritize voice traffic, ensuring minimal delay, jitter, and packet loss. This is typically managed through dedicated bearers set up by the LTE network during a VoLTE call.
• Compatible with VoLTE codecs like AMR-WB or EVS: These codecs provide high-definition voice quality, and their use must be negotiated and supported at both the device and network levels.
• Supports mobility management features: These might include single radio voice call continuity (SRVCC). This ensures that ongoing calls can be handed over seamlessly from LTE to 3G or 2G networks when LTE coverage is lost.

Pros and Cons of VoLTE SIMs

A VoLTE-enabled SIM card unlocks the full benefits of voice over LTE services, but it also introduces some dependencies and limitations. Below are the main advantages and drawbacks of using a VoLTE SIM:

Pros of VoLTE SIMs:

• High-definition voice quality: VoLTE SIMs enable HD voice calls with significantly better audio clarity compared to 2G or 3G networks. 
• Simultaneous voice and data: Users can make voice calls while browsing the internet or using apps, thanks to support for concurrent voice and data sessions. 
• Improved battery life: VoLTE eliminates the need to switch between 4G and legacy networks for voice calls, reducing battery consumption. 
• Faster call setup: Calls initiated over VoLTE connect more quickly than those on traditional networks. 
• Native video calling support: VoLTE SIMs support low-data, network-integrated video calls without needing third-party apps. 
• Wider network coverage: VoLTE operates over modern 4G networks, offering broader and more reliable coverage, especially in areas where 2G/3G is weak or phased out. 

Cons of VoLTE SIMs:

• Device compatibility required: VoLTE functionality depends on having a VoLTE-capable handset. Older or unsupported devices cannot access its features. 
• Limited to 4G coverage: Without 4G or compatible Wi-Fi, VoLTE calls cannot be made. Call quality and connectivity drop in low-signal areas. 
• Inconsistent quality across devices: If the receiving party is not using VoLTE, call quality defaults to standard, negating HD benefits. 

SIM and network configuration dependency: Users must rely on proper SIM provisioning and network-side IMS support, which may not be uniformly available across all regions or operators.

Best Practices for Optimal VoLTE Performance

QCI and DiffServ Marking

VoLTE packets are transmitted over QCI-1, which has a strict delay budget of 100 to 150 milliseconds. If a packet exceeds this budget, it is discarded, as VoLTE uses unacknowledged mode (RLC UM) — there are no retransmissions beyond HARQ. Because of this, it’s critical that network elements treat VoLTE traffic with the highest priority.

To ensure this, the IP transport layer must mark VoLTE traffic with the appropriate DiffServ code point (DSCP), mapping it to high-priority queues in routers and switches. If this mapping is incorrect or not enforced across the transport network, VoLTE packets may experience delay or jitter, leading to poor audio quality. Proper configuration ensures that VoLTE packets avoid congestion and meet the low-latency requirements essential for real-time voice.

Dedicated EPS Bearers

VoLTE traffic requires a dedicated EPS bearer with GBR and predefined QoS characteristics linked to QCI-1. This bearer ensures that voice packets are isolated from other traffic and can be scheduled with guaranteed bandwidth. The bearer setup must happen quickly when a call is initiated; any delay in establishing this bearer can increase call setup time or cause call failure.

VoLTE packets must be treated differently from regular data, not only in terms of resource allocation but also in how they’re scheduled. Failure to provision or maintain these dedicated bearers properly will result in packet drops, jitter, or higher latency, as VoLTE packets could be queued behind best-effort traffic. Ensuring accurate bearer establishment and consistent QoS enforcement is foundational for stable VoLTE performance.

RAN Layer Optimization

In the radio access network, several layers require tuning to support VoLTE’s time-sensitive traffic.

• DRX configuration: VoLTE generates packets every 20 ms. Ideally, DRX cycles should match this interval to provide enough scheduling opportunities. A 20 ms DRX cycle provides about five opportunities to deliver a packet within the 100 ms delay window. However, due to handset constraints — such as with Apple devices — networks often use a 40 ms DRX cycle, reducing the number of opportunities to 2 or 3. This increases the reliance on successful first transmissions and makes DRX tuning a critical optimization area.
• PDCCH robustness: VoLTE success depends heavily on the user equipment decoding the PDCCH correctly to receive its scheduling assignment. If the UE misses this control information, it won’t send a NACK, and the base station may misinterpret the situation as no transmission attempt (DTX). Increasing PDCCH aggregation levels or power for VoLTE improves decoding reliability. Some vendors allow dynamic adjustments of PDCCH aggregation layers based on traffic type, further improving VoLTE performance during DRX cycles with limited on-duration.
• TTI bundling: In poor radio conditions, VoLTE uplink quality can be improved using TTI bundling, where multiple copies of the same packet are transmitted in consecutive subframes. These redundant copies improve decoding chances and reduce uplink packet loss. When enabled, TTI bundling also disables DRX, giving more scheduling windows to the UE, which is beneficial in coverage-challenged scenarios.
• Delay-prioritized scheduling: VoLTE packets nearing their 100 ms delay limit must be prioritized. This requires the scheduler to be aware of packet age and dynamically prioritize older packets to prevent discards. This logic helps reduce dropped calls and improves MOS in congested or fluctuating radio environments.

IMS Optimization

The IMS core controls signaling and session management for VoLTE and must be tightly tuned to avoid delays.

• Uplink grant sizing: Different codecs generate different packet sizes. If the network sends an uplink grant that is too small for the codec’s packet, the UE must request a second grant via BSR, causing delay. Some vendors support adaptive grant sizing, where the network learns the UE’s packet sizes and adjusts grants dynamically. This reduces the need for retransmission cycles and improves uplink efficiency.
• RoHC (robust header compression): VoLTE packets have small payloads (\~60–70 bytes), but IP headers (40–65 bytes) nearly double the total size. RoHC compresses these headers to just 2–3 bytes, reducing the overall size of VoLTE packets. This significantly benefits uplink in poor coverage by reducing resource demands, increasing power per RE (resource element), and improving decoding reliability. RoHC is especially effective when IPv6 is used, where header size is larger.
• SRVCC threshold planning: The network must transition VoLTE calls to 2G/3G (SRVCC) when LTE coverage degrades. This transition must be carefully timed. If the SRVCC threshold is set too low, users may stay on LTE longer than ideal, increasing drops. If too high, SRVCC may be triggered prematurely, possibly before the VoLTE bearer is established (leading to B-SRVCC drops). Aligning SRVCC thresholds with IRAT handover triggers helps minimize these issues. Some vendors also offer timers to delay SRVCC immediately after QCI-1 bearer setup, reducing B-SRVCC risk.

Monitoring and Measurement

Real-time monitoring is critical to maintain and improve VoLTE service quality.

• Packet-level metrics: RTP loss, jitter, delay, and MOS should be continuously measured. These metrics help identify issues in voice quality caused by congestion, poor radio conditions, or DRX misconfigurations.
• DRX and scheduling logs: Analysis of scheduling behavior during DRX cycles can reveal whether missed packets are due to short on-duration windows or missed PDCCHs. Identifying patterns in packet loss tied to specific DRX settings can guide re-tuning efforts.
• Uplink HARQ performance: Since VoLTE uses RLC UM, once HARQ retries are exhausted, packets are lost. Monitoring HARQ retransmission success rates allows detection of weak radio conditions. Increasing HARQ retries slightly can help, though it must be balanced with delay constraints.
• Call traces and failure analysis: Traces should be reviewed to analyze bearer setup delays, failed IMS registrations, codec mismatches, and SRVCC behaviors. These insights help identify IMS-side misconfigurations or coverage gaps impacting VoLTE.

floLIVE: VoLTE SIM for IoT

floLIVE’s VoLTE-enabled SIMs are purpose-built to meet the demanding needs of industrial and regulatory IoT applications that require reliable, high-quality voice communication. Backed by our fully owned IMS core and global multi-IMSI infrastructure, our solution ensures seamless and compliant VoLTE connectivity across borders.

Key capabilities include:

• Global VoLTE coverage through intelligent multi-IMSI switching, enabling regional voice support in North America, Europe, and Asia-Pacific. 
• eCall readiness for automotive use cases, including SIP signaling, MSD transmission, and emergency prioritization via QoS. 
• Voice bundles and flexible billing models, including per-minute, pooled minutes, or customized plans based on your deployment needs. 
• Support for mission-critical voice in verticals like connected cars, lone worker safety, smart elevators, security systems, and telehealth. 
• SIM form factor flexibility, including physical SIM, eSIM, and iSIM with VoLTE-ready profiles.

Whether you’re building a connected vehicle platform, enabling emergency call systems, or deploying voice-enabled IoT devices, floLIVE’s VoLTE SIM delivers the voice quality, regulatory compliance, and flexibility your use case demands.