Phone Cuts Off During Calls and Jumps to Facetime & Speaker Phone

My Mom's IPhone 4, during phone calls, constantly cuts off (doesn't register as dropped calls though,) jumps into facetime and jumps into speaker phone for no known reason. She has taken it to the Apple Store Support Team three times. The first time she was not treated nicely and was told nothing was wrong, the second time I went with her and they wiped her phone clean and started from scratch and the third time she bought a brand new phone. None of these options have worked and the problem persists. This is her fourth IPhone over three years of no major issues until now, so I don't see why this would just start happening for her in the last year. Are there similar issues like this in the community and does anyone have an idea how to solve it?