T.30 or T.38
T.30 is an International Telecommunications Union (ITU) recommendation that defines the procedures for transmitting a document between two devices over the public switched telephone network (PSTN). T.30 dates from before the advent of the internet. More recently, Voice over IP (VoIP) has become a popular alternative to the conventional PSTN, and this has given rise to fax over IP.
Conventional fax machines were not designed to deal with the vagaries of IP networks that impair the data stream, such as latency, jitter and packet loss (see below). And so the T.38 recommendation was devised as a way to transport fax data across IP networks and overcome the problems that IP presents. However, in practice, it seems that the vagaries of IP are not as bad as they might once have been, and T.30 faxes are routinely sent over IP with very good results.
Choosing between T.30 and T.38 can come down to experimentation. Which of the two will work best for you might simply depend on the quality of your telephony provider's IP link. Also, find out whether your telephony provider supports T.38, not all of them do.