Hello weavos,
Sorry to hear about that fault, have you tried reporting this to BT? You can test your line for faults online by going to www.bt.com/business/fault however this will not work if you have a OneBill account, or an ISDN line.
Can you give us some more info on this? Do you know if the line is a standard PSTN line, a featureline, etc?
TheOtherOne