After trying several different (working) phones, it appears I don't have a working landline.
Trying to use the landline help pages on the website has been difficult as it either assumes the phone is at fault, or gives advice on how to set up the new phone-via-router devices which I don't have/do not believe the SS13 area has been migrated to yet.
I've checked my number and it appears that the line has been stopped and cannot be reactivated, so I cannot port the number to another provider (assuming I wanted to). I've checked my correspondence and I've not been notified of the number being stopped, or that there were any issues, all my payments are up to date and I'm left without the landline I need for my work.
How can I move forward? I need the number and landline to be up and running so my work calls can be routed to me, and they need to be routed to my landline and not a mobile so I'm a bit stuck at the moment.