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
Automatic Fax Reception
When configuring your incoming rule, make sure that T.38 support option is selected.
Send or Receive T.38 fax
Make sure that T.38 FAX option is enabled in the main window.