I have been having this problem with my phone. It only happens in my area and doesn't do it anywhere else. Whenever i make a telephone call its fine for a few minutes then the other person on the line said it sounds like I've gone into a tunnel then after a few minutes it comes back to normal. Then it will happen again. Basically it sounds like i go in the distance.
I have tried two mobiles so i don't think its my phone. I have a S7 Edge and the signal is very strong. I have full 4G and full 3G. I have turned 4G off and tried 3G and vice versa. It still breaks up.
My partner is on BT Mobile too and hers is fine. Any idea why this happens? I can't make any phone calls without me breaking up to anyone. I even called my bank and they said same. As mentioned i have very strong signal. Constant full bar on all 2G/3G/4G.
I have raised a complaint as you have suggested and will report back. The agent said there was a problem in the area of my postcode, however, this has been happening for months. Hopefully they can look into this. My friends are in the same area and same network, but theres doesn't break up! how is this possible?