As Voice over IP (VoIP) replaces office telephone lines with SIP Trunks, there is still one function that requires the old analog phone lines: FAX. Many businesses use faxes due to its ubiquity, ease-of-use, and legal status.  However, supporting analog fax lines and debugging fax issues on VoIP phone systems is a major headache for System Integrators (companies that install and support office PBX and handsets) and Internet Telephone Service Providers (ITSPs).  

The problem is the standard "FAX" transmission.

Standard FAX

Traditional FAX machines worked on a Group 3 (G3) protocol that compresses the image for transmission.  The recipient's Fax Machines would then uncompress the transmission and print the image.  This required a lot of signaling data between the two fax devices to ensure all packets were received and put back together in the right order. 

Supporting traditional G3 fax was difficult for some Softswitch vendors, but not for Sippy.   Sippy Softswitch is a back-to-back user agent solution, meaning that the packets that come into Sippy, go out of Sippy - untouched and as such, standard G3 FAX is supported over the Sippy Softswitch.   

T.38  Fax over IP (FoIP)

In a typical VoIP installation, analog phone lines for fax are provided by an Analog Telephone Adapter (ATA).  The ATA connects to the IP network; it detects when a fax call is starting and converts the analog tones of a normal fax call into the T.38 network packets that can move along the IP network.  The ATA may also be built into a multifunction printer (MFP).

Some Fax devices (such as MFPs) and T38 compatible fax machines, transmit directly as T.38 and can receive and convert inbound calls to an image, and then print as normal.   An image of a sample FoIP solutions is shown at the end of this article.

A known issue has been reported in our production releases that may affect the successful transmissions of fax in this method.  This occurs when a Vendor starts sending RTP in g711 after issuing a re-INVITE for t.38.  In this case the softswitch does not relatch to the revised port that is sending the media.  This issue is known and maybe corrected in future releases.


Service providers (such as companies like J2)  convert faxes to emails, allow people to send a document (such as a word document) as a fax.  Most e-fax solutions convert the document to a regular fax format to send to regular fax machines.   This requires having a Front-End-Process (FEP) to transmit the document as a fax or convert a fax transmission to a document.

Sippy does not provide an "eFax" solution - but does provide all the APIs needed for a partner to develop an eFax solution that integrates with Sippy.   APIs can be used to synchronize billing data between the eFax system (and users) and the Sippy Softswitch.  The Sippy solution can then be used to invoice the eFax customers, modify balances, perform top-ups etc.