The VoIP Forum is dedicated to enabling and promoting industry-wide interoperability between products that send telephony over the Internet and private IP enterprise networks. This interoperability will allow seamless transmission of Internet phone calls regardless of the telephony application used by callers and call recipients. At present, applications from different vendors are incompatible due to fundamental differences in voice coding, silence suppression, addressing and dialing plans, call management and other related functions.
Source: wiktionary