There are many different standards
and groups of standards in Internet Telephony.
The most common groups are: developing standards, H-Series, T-Series, and
G-Series. Furthermore, the most
popular voice-over IP protocol is the H.323 protocol.
H.323 is a "standard" protocol for providing all sorts of
interactive multimedia communication. It was originally designed primarily for
video-conferencing, but it really consists of a complicated platform for all
sorts of multimedia content. Here is a link to a list of Protocols.