Some 4G phones don’t support 4G calling correctly or with all networks.
Check whether your phone actually uses 4G (VoLTE) for calls – to sort-of test this, make a call while connected to mobile data and note whether the signal meter’s 4G indication drops to 3G(H+/HSPA) or something during the call.
A more insidious version of this issue is that some phones do VoLTE just fine, but only use 3G for 000 calls.
Some phones only support VoLTE with some networks. There’s a chance that an appropriate firmware upgrade could add support for some network/device combos.
Yep, this is me. Bought 1 yr vodafone prepaid, after getting reminded to enable VoLTE on my phone (a 2019 model), which I did. But, the phone doesn’t actually do VoLTE calls, so come Vodafone’s 3g shutdown weeks later I suddenly could no longer make or receive ANY phone calls. Internet/data works fine.
Ended up getting a cheap new phone just so I can make phone calls in an emergency.
