T.38 is a protocol specifically for transmissions of faxes. This is done via UDP and / or TCP within a SIP session. This mode of transmission is much less sensitive than sending a fax in the audio channel of a SIP call. The real-time behavior has a negative effect on the error-prone fax connection. If you get lost using G.711a codec, so uncompressed calls and some language or arrive later, take a crack at most true. For a fax that's the demolition. T38 prevents this by an active control protocol which receipts for each data packet sends and receives.
You can configure where you can register your T.38-capable hardware in the customer fax connection. These include, among others, the AVM Fritz!Box. As soon as you will be working with this connection a call automatically treated as a fax call and you can be sure that your fax will also be sent easily. Incoming faxes are also enabled on the connection of T38 has passed. You have the choice between incoming and outgoing T.38 or T.38 only outgoing and incoming fax-to-email.