MWEB Faxmail Premium doesn't work

I have subscribed to the MWEB Faxmail Premiun service for many years. A while ago, I stopped receiving incoming faxes. On further investigation, I found that my incoming fax number was always engaged (i.e. BUSY tone). I tested this from several locations, at different times of the day. I reported it to the MWEB technical support line 6 or 7 times over the past couple of months. (The most recent reference number was MWB***9.) Each time I was told it would be investigated, and each time nothing happened - the fault is still there. Needless to say, I am still being billed for a service which doesn't work!