OnePlus
Enable or Disable VoLTE Calls auf OnePlus
VoLTE (Voice over LTE) routes phone calls through your 4G data connection instead of the older 2G or 3G voice network. On OnePlus phones running Android with OxygenOS, this setting sits per SIM card, so dual-SIM users need to check both slots separately.
When VoLTE is off, your phone drops to 2G or 3G every time you place or receive a call, which explains slower call setup and lower audio quality. Turning it on keeps the call on the 4G network whenever coverage allows, giving you HD voice and a faster connection.
To change this, go to Open the Settings ➔ Tap on Wi-Fi & network ➔ Tap on SIM & network ➔ Choose a SIM card ➔ Enable or disable VoLTE. Repeat the steps for the second SIM if you use dual SIM.
Leave VoLTE off if your carrier does not support it in your area, since forcing it on without network support can cause calls to fail or fall back automatically anyway. Keep in mind that VoLTE calls draw less battery than constant switching between 3G and 4G, but the option only appears if your SIM and plan actually support it.