T68i Fax Problem?

This phone has suddenly stopped sending faxes. It can receive faxes
okay and make DUN connections to my IP. But when I try sending a fax
via the Windows Fax Client, the phone attempts to connect then bales
out with a software message 'number is busy'.

T68i will also not send faxes with MIGHTYFAX or Classic Phone Tools.
In the latter software the message reads 'number is blacklisted'.
Needless to say this applies to neither my T68i number or any of the
numbers I am calling.

Any suggestions most gratefully received.