I'm not sure I've come across a fixed line phone that needs to be registering.
I'm inclined to, first of all, suspect the phone itself. If you plug it into the main one downstairs is it still permanently registering or is it ok ? Trying to work out if the issue is the phone or the wall socket/connection.
If it is the phone, then you need to repair/replace it.
If it is the socket, did BT put it in for you or did a third party do it ? If BT then you may have a claim to a free repair, but you'd need to check, obviously. If a third party then BT won't be interested and you'd need to get someone to check out the connections.