The PSTN is the system most of us are accustomed to where calls are routed according to a telephone number and its components, including the country code and area code. This is usually to acheive voice calls, but since any data can be encoded as audio it can be used for more general purposes, like faxes. The PSTN and the rules for how it works are mostly set by the ITU.
In a lot of ways, this is quite like the Internet, where identifiers are mapped according to DNS, connections are routed via IP, and it can be used for a wide variety of purposes. There are some important distinctions to be made, however:
.comdomain names, say, can be chosen on an individual basis.
It has been recognized within both the public and private sectors that the telephone network is to be succeeded by VoIP, a catch-all term for any and all systems of Internet telephony. A system called ENUM is designed to handle the transition by mapping phone numbers to DNS records that store an individual's digital identifiers, such as email and VoIP addresses.
During the George W. Bush administration, it was said that this convergence between the telephone network and the Internet enabled by ENUM could in fact be done in tandem with expanding broadband access. This system would allow a smooth migration from the use of existing phone numbers to digital identifiers. However, it was decided that with respect to this technology, industry ought to lead the way undisturbed. There is little incentive for telecommunications companies to offer to their customers a system that allows them to ween off their services. It has been so observed that in Australia, for example, ENUM has mostly been ignored. Likewise, no activity on ENUM deployment has taken place in the United States since the ENUM trials in 2007.Greater awareness of SIP and XMPP is needed to increase their adoption. Fortunately user agents have caught up, especially for XMPP, and it's now very easy for a new user to get started.