If one wire of your phone line is disconnected, you will get a very poor speed, and no dial tone.
You need to report it as no dial tone. Do not mention the broadband.
Once the phone fault is fixed, then the broadband will get better. It will get fixed quicker, as there are plenty of Field Technicians who can fix phone faults, but not many broadband ones.
Check at the test socket first.
As a maatter of interest, I have had several 'line cord error' showing up on my BT 8500 phone (land line) whilst the broadband was just fine.
The error just goes away after a while.
I did speak to a BT phone engineer a couple of years ago and he said it was a line error that should clear itself.
It is, I think, only BT phones that have the 'error response' built in.