callcentric.com is a popular SIP provider in the United States. Callcentric supports both T.38 and audio faxes. This service provider also offers phone numbers in the US/Canada and many other countries. To get an account you can register at the Callcentric website.
Configure SIP account
Username (or SIP-ID): the phone number provided by the Callcentric
Password: password
SIP Registrar Server: callcentric.com
Configure Outgoing Call Routing
On the General page, in the Dial on Line area, check that the following settings are used:
VOIP Protocol: SIP
Dial on SIP Server: callcentric.com
Username: the phone number provided by the Callcentric
On the Fax page, in the Fax Support area, make sure that Send T.38 re-invite option is selected.
Configure Incoming Call Routing
On the Fax page, in the Fax Support area, make sure that T.38 fax option is selected.