Phone has 4g data, can't make calls... That's a VoLTE issue.
Since 4g, phone calls are now actually data, not calls. For a phone to make a call over 4g and 5g, they need to support VoLTE and have it toggled on. If this is not on, calls will not work because 3g is/has been shut down.
(Edit, 2g will still function as a backup, calls will try to use that where possible. 2g is old and being shut down in a few years so it's not a long term solution. This will also mean while on a call, data will be 2g, so basically not exist)
Hopefully the fix is as easy as toggling VoLTE on in your settings. But you have a rather old phone, if that phone doesn't support VoLTE, not much you can do.
To explain in a little more detail. 2g and 3g have data and calls. Separate sorta channels using the sorta same system as how landlines work. 4g modernised things by making phone calls basically the same as a whastapp/skype/facetime call. Sends the data over the internet. To do this, the phone needs to actually know how to do that. Before this existed, phones would swap to 3g when you made a phone call, then swap back to 4g when the call ended. Issue is, the uk is shutting down 3g. So if your phone doesn't support VoLTE, it'll try to use 3g to make a call with the old system, 3g is gone, call fails.
(So to be short. Phone should work as long as it can get a 2g signal. That's not ideal and has it's own issues but it should still work for a few more years)
