It might not be relevant to you, but this may help someone with a similar problem. A few years back when still on pstn we had the same issue - but a different resolution. In our case it turned out one of the corded extension handsets was faulty, and was bizarrely causing problems with all the handsets - we know it was the handset because it caused the same problem plugged into the main phone socket. Discarded that handset and everything else worked perfectly!
