There are a number of reasons why you will lose signal when using the telephone. The most common is a missing broadband filter from an extension socket with a telephone connected. Check that all devices connected onto the telephone line have filters, including alarm systems and Sky boxes which are often overlooked. If you have checked this and found all filters in place, there is a chance the fault is external to your property and the responsibility of BT. Please contact me to discuss the issue, as I can prove and repair the fault if not in the BT network.