Wifi calls work fine, cellular calls do not

[Ellie010809]

My mom has an iPhone 6s. A couple of weeks ago her phone suddenly stopped working properly. When she makes or receives calls on cellular, there is some static but her voice does not go through the call but echos back in the phone’s speaker. She went to the Apple store and switched out her SIM card at VZW but no one gave her answers. Trying to get a little more info before I take her back to have it looked at. 

Here is the troubleshooting I’ve done so far:

Wifi voice calls work normally. (Voice can be heard.)
FaceTime on both wifi and cellular work normally.
Voice memos and video recording work normally

There are no obvious problems like the mute button being on. Network settings have been reset. Phone had a hard reset. Phone was then reset to factory settings and backup restored to phone. 
iOS up to date. 
The microphone hardware seems to be in working order given the above so I’m assuming this must be a software issue. Am I correct in this assumption? Has anyone had this issue? Should I reset phone and start new to avoid installing the last couple of updates?