You might need to dial an extension to reach a fax number. FAX.PLUS makes it easy to send fax to numbers with extension.
To send a fax to an extension go to the send fax tab, enter your number, followed by a * and then the extension you wish to dial. (Each * represents one second delay, and you can enter up to 10 stars).
(Dials +16692001010 waits 4 seconds and then dials 5)