The step by step explanation is
here .
It does not matter at all, with what you want to send or receive a fax, being it a fax machine, an analog modem or an isdn modem. The unit used must comply to the specification laid down in the G1 (old) G2 (still old) or G3 (todays standard) rules.
Basically sending an dreceiving fax is a simple point-to-point connection with it's own transfer protocol.
And an ISDN modem does it (normally) at 14.400 according to G3 standards.