Xperia 5 III only making calls on GSM network

[nznick]

Have had phone for a few months and everything was working fine. But suddenly now I cannot make or receive calls when connected to 3G or 4G networks. Data and SMS still works ok on 3G and 4G but I can only make or receive calls when I change the Preferred network type to 'GSM only' in the SIM menu. The sim card works fine when I try it in another phone and other working sim cards replicate the same problem when I use in the Xperia so the problem must be in the phone somewhere. Can someone assist here with a solution? Thanks.

[Piscie]

Hey @nznick, have you updated your phone to Android 12? You also need to set your device to only use either a WCDMA (3G) or GSM (2G) network.

As per the support page, the device automatically detects the fastest network available. If the signal strength of the fastest network is unstable, the device will keep switching back and forth between mobile networks and that will lead to an unstable connection as well.