In reply to a post by FarmerGilesUK:

My ISP and voice provider is PlusNet.

Disconnect all broadband equipment form the line and plug a (preferably corded) phone into the test socket, on the wall at the back in this pic.

Dial 17070, Option 2, and if it isn't silent then Plusnet are required to raise a voice fault with BT. So insist. If it is silent, then either your wiring or equipment is dodgy.