iphone6 when I answer a call it is just silent on both ends

[gsdayton]

i phone 6
When I answer an incoming call from one specific person, there is just silence on both ends.
This is only happening with one caller. It happens when she calls me and when I call her.
It is not happening with any other caller. We can text but can't talk.
I am new to Straight Talk. She is a long time Straight Talk customer. We both recently got new phones.