One of the toughest challenges in VoIP is setting up communications which are not voice related.
Sending/receiving faxes over a VoIP network, is such a challenge.
There are 3 ways to tackle this issue:
1/ use web2fax and fax2mail: on our V1 and V2 platforms , you can use web2fax and mail2fax. These allow the customer to upload a pdf file, and send it to a specific faxnumber for outbound. For inbound, a number can be configured to send a fax directly to an email address, or to a mailbox. The latter stores a copy on the machines of 3starsnet.
2/ use passthrough. In many cases you can get a reasonable result using g711 to transmit a fax. As faxes do have quite some error correction in the protocol, you can get a reasonable successrate in transmitting faxes. This can be applied on our V1, V2 and V3 platform. The typical setup would be a ATA (analogue telephone adapter), which is connected to a fax machine.
3/ use t38. A specific protocol was designed to make the transmission of faxes more reliable over VoIP networks. Comes in T38. Our V3 platform has support for T38, which gives the highest reliability you can get to in fax transmission on VoIP networks. Good results are got with Grandstream and Cisco. But we also have positive responses over Patton and 3cx. Not, only our V3 platform supports this.
Still, for very intense fax users, it's not to be advised to use VoIP for this. T38 can be used to reliably send faxes of upto 10 pages, but not for fax transmissions of tens of pages.