Our phones are of the plug and socket type and all of them are affected, plugging one directly into the test socket at the master box had no effect. But to my surprise I found that my Talktalk broadband service which uses the same BT land line as our phones is working ok.
I submitted an online report to Talktalk (our phone and broadband provider) and its response was to link me to its official report process...but guess what that process requires before it will allow you to submit it? Yeah, a working phone number to enable updates to be sent. You couldn't make it up, lol.