What exactly is happening here with my elderly dad's phone?

Dad signs up for AT&T with this 2016 cheap phone. The issue is with calls in particular at home.

Whenever he calls (or receives a call) the phone drops from LTE to 3G instantly. Sometimes he can make the call, other times no. Same with receiving, sometimes the caller instantly gets a voicemail message. As soon as the call is done, the phone kicks back to LTE.

When he is on LTE, data is 100% fine and fast. If he drives around, the ability to call is increased, when dropping to 3G but still sometimes spotty.

My thought was, he is clearly getting the LTE signal at home just fine, but maybe the 3G which it drops to when he goes to make a call is weak. Maybe there aren't any close 3G towers anymore (idk if I'm making this up or not)? The phone doesn't appear to have a VOLTE option (I assume AT&T has this in general, I'm a Verizon guy).

Any thoughts? I know its easy to say upgrade to a new phone, but he is very stubborn about it unless I can prove before he purchases it that its a phone problem and not an AT&T one.