iPhone 16 not ringing for incoming calls on Xfinity Mobile

My wife has an iPhone 16, on Xfinity Mobile. She previously had an iPhone SE, also on Xfinity. As the SE aged, some phone calls failed to ring on her phone. Some went to voicemail, others didn't. Finally, Xfinity told her that a newer phone would solve this, and that's why she got the 16.

This did NOT solve it. We took it up with Xfinity but they couldn't account for it, and some calls still don't ring. A call from me, clearly not an unknown number, failed to ring today, although the phone did vibrate.

Some of the calls she's missed were important, eg from Dr's offices. We appreciate any and all suggestions.

FWIW-- my phone is a Samsung Galaxy (don't shoot me!) on the same Xfinity network, and I never have this problem.