Always worth swopping with a known good example. Problem with phones these days is that issues can be the phone itself, it can be your internal wiring, the external wiring, the exchange, anything really. And of course you are in the frame to pay the charges should the issue be investigated, as it should be, and the problem found to be in your property. It's not a good system any more. IMO no cost problem identification should be part of a phone service operator's licence.