[Paul]

Well if you can get hold of another phone, plug it into the line and if you get the same fault you are in a good position when you ring in as you can then say....
"No it's not my phone as I have tried a phone which works perfectly elsewhere and the phone showed the same fault when plugged in here". Is anything else plugged into the line? Another phone etc? If so unplug everything except 1 phone and see what happens.