Why does my elderly mother's iPhone keep going to voicemail despite checking all settings?

I have checked everything over and over again! From silence, unknown, callers, to call, forwarding, to whether the volume is turned all the way down, to whether it is set to go to voicemail directly in the phone, to looking and scrolling through every blocked number in her iPhone. My mother is elderly… So this is becoming a serious problem! Can anybody please help? We actually traded her last phone in for this problem and cannot believe we are back to square one with this current iPhone in less than one week… We seem to have the same problem. And why her phones? What is it about her phone number as a Verizon customer that this keeps happening to when it is not happening to any other iPhone user in a pretty darn large family.

[Re-Titled by Moderator]