That beep during calls could be caused by several different things, some simple and embarrassingly easy to remedy

:
Do you have call waiting? If so, the notification beep for that might be set too high for you (easily tested, as you'll have a missed call

).
Is it happening with just one person you're calling? If so, it could be just them accidentally pressing a key during the call, if it's not a cell phone you are dialing to.
Also, sometimes a brief feedback loop can happen if the receiving volume is too high on your device.
If the volume button on your device is being accidentally pressed during a call, a loud beep can come into the speaker, also. Some users have reported accidentally holding the volume down button to the point of it automatically indexing up, causing a loud beep.
You'll need to think through all the possibilities, because it is very doubtful that it is caused by the carrier.