says BT Hub 5A

we have two phone lines. One for the broadband and the other for telephone

Ok. What I'm getting at is that you may have a noisy line which is often caused by poor extension wiring or a line fault. It sounds like you may not have any extension wiring so you should report the problem and get an engineer to visit and find and fix the fault.

Be aware that if the problem is due to your wiring you may be charged £150 by BT.