T.38 is used to transfer faxes over (data) networks in realtime. T.38 converts the analog fax signal into an image and transfers it via the network to a T.38 compatible device. If this device is an IP phone system or a gateway, it might transform it back from T.38 into an analog signal and send it via PSTN (the public telephone network) to an analog fax machine. If you are using a VoIP/ SIP provider which supports T.38, the phone system doesn’t need to transform it back into an analog signal and sends it in T.38 format. Of course, this also works the other way round.
Many VoIP gateways already support T.38. A growing number of SIP/VoIP trunking provider also supports T.38.
Askozia IP phone systems have full T.38 gateway capabilities. Additionally, you can create virtual faxes which convert incoming faxes into PDFs and send them via email. This way no fax machine is required for receiving faxes.