Before VoLTE became the standard, a 4G phone making a voice call had to use one of two legacy mechanisms. This page compares all three approaches.
CSFB was the interim mechanism deployed while operators built out IMS infrastructure for VoLTE. When a 4G device received a voice call or initiated one, the network instructed the device to drop to 3G (or 2G if 3G was unavailable). The call was then handled as a traditional circuit-switched call on the legacy network. When the call ended, the device returned to 4G.
Problems with CSFB:
Traditional 2G and 3G voice used circuit-switched bearers – a dedicated time slot or radio channel reserved for the duration of the call. This was reliable but inefficient: the channel was occupied whether or not speech was present. Voice quality was limited by the narrowband codecs (AMR-NB on 3G).
| Parameter | 2G/3G Circuit Voice | CSFB | VoLTE |
|---|---|---|---|
| Call setup time | 3-5 seconds | 6-8 seconds (+ fallback) | Under 2 seconds |
| Voice codec | AMR-NB | AMR-NB | AMR-WB (HD Voice) |
| Audio frequency | 300-3,400 Hz | 300-3,400 Hz | 50-7,000 Hz |
| Data during call | Limited (HSDPA) | Drops to 3G speed or off | Full LTE speed maintained |
| Battery impact | Low | Higher – two handovers | Lower – stays on LTE |
| Available in UK | 2G only (no 3G since 2024) | No – 3G networks closed | Yes – all 4 operators |
| IMS required | No | No | Yes |
| SIM ISIM required | No | No | Yes |
This is worth being explicit about: CSFB is no longer a fallback option in the UK. It depended on the existence of an active 3G network. All UK 3G networks were switched off in 2024. A device configured to use CSFB on a UK network will attempt to fall to 3G, find no 3G available, and fail to complete the call. The only voice mechanism on UK 4G networks is VoLTE.