My BT landline lost dialling tone on both DECT phone and corded phone and would not accept incoming calls. It was the same in the master socket (ADSL filtered faceplate) and in the test socket. Infinity FTTC was unaffected running on a filtered extension.
BT online help wanted me to connect my Homehub 5 to the test socket and connect to the internet. This I did, and as forecast phones became fully functional. I am happy that it worked but puzzled as to how it could work. Any suggestions appreciated.