FAX was designed for analog networks, and can not travel over a digital VoIP network. The reason for this is that FAX communication uses the analog signal in a different way to regular voice communication. When VoIP technologies digitize and compress analog voice communication it is optimized for voice and not FAX signaling. Subsequently, there are a number of things you need to take note of when you move to a VoIP Phone System.
If you want to continue using your old fax machine, and you want to connect it to your VoIP phone system, its best to use a VoIP Gateway and an ATA (Analog Telephony Adapter) that supports T38. T38 is a protocol designed to allow fax to “travel” over a VoIP network.
It is also possible to convert to computer based fax and choose a VoIP phone system that supports fax. 3CX Phone System for Windows includes a full featured fax server that is able to receive faxes and forward them in PDF format to e-mail.
Another way to deal with fax when you switch to a VoIP phone system is to connect the fax machine directly to the existing analog phone line and bypass your VoIP system.