Voice Over Internet Protocol uses codecs to sample, compress, and decode in such a way that are optimized for voice. But that optimization for voice means other communication media are incompatible. Many businesses rely on fax machines, and fax machines are designed in a way that they cannot read data the same way a computer with a modem can. That is why the “T.38” protocol was implemented. The T.38 protocol is a standard for fax machines in which the sender, the receiver, or both the sender and receiver are using VoIP.
The implementation of an internet fax can be tricky. One Fax Over Internet Protocol (FoIP) is “store-and-forward,” (T.37) in which the entire image, which is compressed at the source, is decoded at an intermediate site, then sent, in whole, to the receiving fax machine. The biggest drawback to this method is that it is not in real time. Another ideal situation is to have both the sender and receiver use T.38 Fax Machines, also called “internet- aware fax machines.” But, the most common scenario is that at least one component in the chain runs, or was designed to run, on the POTS network.
A T.38 gateway is a device that converts the faxed image into data that can then be encoded, or takes the data that is encoded and converts it to an image that a fax machine can read, and then print out. It is entirely possible that two offices that are both using Voice Over IP for their phone lines will have older fax machines, and both offices will have gateways. Many VoIP providers offer an Internet Fax (alternately called CloudFax, Virtual Fax, etc.) service, which you can use to send or receive fax over their network as an email.