Connection ok but Pages not faxed

I have just set-up the fax capability.
When sending a fax, the status box and audio (fax whine) confirm that
dialing is ok and a connection is made (the person I'm faxing to confirms
thier machine picks-up and receives notification of an incoming fax). The
status box then states 'sending page one of one'.
But nothing is received and I then get an error message (no details) that
the fax did not get sent.
Any ideas.....?
Cheers
Annie