Thanks for your post and welcome to the Community Forums, arbuthknott,
Sorry to hear that you have been having issues with your landline. 21CV cannot be setup from your end. It will require you calling in and speaking with a member of the team for them to provision the service for you. They will need to ask you specific questions relating to your landline and ensuring what dependencies you have on the landline before converting you to 21cv service. Then it may require an engineer to come out and set things up for you.
Regarding the current issues that you have having with your landline, I've run some remote diagnostics and have not been able to identify any issues with your connection. If you need I can make arrangements to get an engineer out, however if you are planning to convert to 21 CV then I'll hold off on sending someone out as you would be making a switch to the servce.
Cheers,
Corey C