The GlobeSurfer era was pure data - 3G voice fell back to GSM circuit-switched calls. Modern 4G and 5G networks carry voice as data too, alongside a stack of specialised services from IoT connectivity to network slicing. This guide covers the standards that define what mobile networks actually do beyond basic data.
VoLTE (Voice over LTE) carries telephone calls as IP packets over the 4G data network rather than falling back to a legacy 2G/3G circuit-switched connection. It uses IMS (IP Multimedia Subsystem) as the core signalling architecture.
The practical improvements over legacy voice are significant. VoLTE calls set up faster - typically under 2 seconds versus 6-8 seconds for circuit-switched. Audio quality is higher using wideband codecs (AMR-WB at 16kHz) or the newer EVS (Enhanced Voice Services) codec which extends to 20kHz - noticeably better than standard narrowband voice at 3.4kHz.
VoLTE also enables simultaneous voice and high-speed data on a 4G connection without the speed drop that occurred when 3G handsets fell back to 2G for calls. And with HD Voice plus the ability to send video during calls, it forms the foundation for the full RCS (Rich Communication Services) experience.
VoNR (Voice over New Radio) is the 5G equivalent - voice calls carried over the 5G NR radio access using a 5G SA core network. VoNR requires 5G Standalone architecture (not NSA) and delivers lower latency than VoLTE - important for real-time voice quality.
UK rollout of VoNR is tied to 5G SA deployment. Three UK was first to deploy 5G SA commercially in the UK. Other operators are following through 2024-2025.
Wi-Fi Calling (standardised as IMS over Wi-Fi or VoWiFi) uses a broadband internet connection to carry calls through the operator's IMS core. When mobile signal is poor indoors, the phone seamlessly hands off to Wi-Fi Calling. For router users, this means a good 4G/5G router connection can provide indoor phone coverage even with poor direct cellular signal - the phone uses Wi-Fi to reach the operator's network over the router's cellular backhaul.
| GSM Voice (2G) | AMR-NB codec, 3.4kHz bandwidth |
| 3G Circuit Voice | AMR-NB, falls back to GSM |
| VoLTE | AMR-WB (HD Voice), 16kHz |
| VoLTE + EVS | EVS codec, up to 20kHz |
| VoNR (5G) | EVS, ultra-low latency |
| Wi-Fi Calling | VoLTE quality over broadband |
UK operators are progressively switching off 2G and 3G networks to reclaim spectrum for 4G and 5G. Vodafone closed 3G in January 2024. This makes VoLTE mandatory for any device making calls - devices without VoLTE capability lose voice service as 3G is withdrawn.
NB-IoT (Narrowband IoT, standardised as LTE Cat-NB1/NB2) is a 3GPP standard designed for low-data-rate IoT devices - meters, sensors, trackers. It operates in a 200kHz channel within existing LTE spectrum and is optimised for power efficiency over raw speed. A battery-powered NB-IoT device can achieve 10-year battery life on a small cell.
NB-IoT delivers around 26-66 kbps - far slower than LTE but entirely adequate for sending meter readings, sensor telemetry, or asset tracking data. Coverage penetration is high: NB-IoT can reach deep into buildings and underground locations where standard LTE struggles.
LTE-M (LTE-MTC, Cat-M1) is a higher-capability IoT standard than NB-IoT - up to 1Mbps, with voice support (VoLTE over LTE-M), and mobility support making it suitable for moving assets. It operates in a 1.4MHz channel within LTE spectrum.
The choice between NB-IoT and LTE-M depends on the application. For static sensors sending small packets: NB-IoT. For mobile assets, voice-capable IoT devices, or applications needing higher throughput: LTE-M. For full-fat IoT data connectivity, standard LTE Cat 1 or above - available with IoT SIMs from iotsims.co.uk.
Network slicing is a 5G SA capability that allows operators to partition a single physical 5G network into multiple isolated virtual networks - each with its own performance characteristics, security and QoS guarantees. A slice can be configured with guaranteed throughput and latency for an FWA customer, different parameters for a mobile broadband user, and yet another configuration for an industrial IoT deployment.
For FWA users, network slicing enables operators to commit to consistent speed levels regardless of wider network load - a significant improvement over best-effort LTE services. It is one of the key commercial differentiators that operators use to justify 5G FWA as a genuine fibre replacement rather than a mobile broadband workaround.
| Standard | Purpose | Speed | Use Case |
|---|---|---|---|
| LTE (Cat 4-18) | Mobile broadband | 150Mbps - 1.2Gbps | Phones, routers, CPE |
| VoLTE | Voice over LTE | N/A (voice service) | All 4G capable devices |
| NB-IoT | Low-power IoT | 26-66 kbps | Sensors, meters |
| LTE-M (Cat-M1) | Mobile IoT + voice | 1 Mbps | Wearables, tracked assets |
| 5G NR Sub-6 | Capacity + speed | 500Mbps - 2Gbps | FWA, mobile broadband |
| 5G NR mmWave | Ultra-high capacity | 2-4 Gbps | Dense urban FWA |
| VoNR | 5G native voice | N/A | 5G SA networks |
| Network Slicing | Virtualised QoS | Configurable | Enterprise, FWA SLA |