The most convenient route between telephone service providers is through the PSTN, since you can’t offer phone service without connecting to it. Because of this convenience telephone service providers tend to consider PSTN connectivity adequate, and don’t take the additional step of delivering IP connectivity. This is unfortunate because it inhibits the spread of high quality wideband (HD Voice) phone calls. For HD voice to happen, the two endpoints must be connected by an all-IP path, without the media stream crossing into the PSTN.
For example, OnSIP is my voice service provider. Any calls I make to another OnSIP subscriber complete in HD Voice (G.722 codec), because I have provisioned my phones to prefer this codec. Calls I make to phone numbers (E.164 numbers) that don’t belong to OnSIP complete in narrowband (G.711 codec), because OnSIP has to route them over the PSTN. If OnSIP was able to use an IP address for these calls instead of an E.164 number, it could avoid the PSTN and keep the call in G.722.
Xconnect has just announced an HD Voice Peering initiative, where multiple voice service providers share their numbers in a common directory called an ENUM directory. When a subscriber places a call, the service provider looks up the destination number in the ENUM directory; if is there, it returns a SIP address (URI) to substitute for the phone number, and the call can complete without going over the PSTN. About half the participants in the Xconnect trial go a step further than ENUM pooling: they interconnect (“peer”) their networks directly through an Xconnect router, so the traffic doesn’t need to traverse the public Internet. [See correction in comments below]
There are other voice peering services that support this kind of HD connection, notably the VPF (Voice Peering Fabric). The VPF has an ENUM directory, but as the name suggests, it does not offer ENUM-only service; all the member companies interconnect their networks on a VPF router.
Some experts maintain that for business-grade call quality, it is essential to peer networks rather than route over the public Internet. Packets that traverse the public Internet are prone to delay and loss, while properly peered networks deliver packets quickly and reliably. In my experience, this has not been an issue. My access to OnSIP and to Vonage is over the public Internet, and I have never had any quality issues with either provider. From this I am inclined to conclude that explicit peering of voice networks is overkill, and that if you have a VoIP connection all that is needed for HD voice communication is to list your phone number in an ENUM directory. Presumably the voice service providers in Xconnect’s trial that are not peering share this opinion.
Xconnect’s ENUM directory is enormous, partly because it is pooled with Pathfinder – the GSMA ENUM directory administered by Neustar. Xconnect’s ENUM directory had over 120 million numbers in it as of 2007.
Xconnect and the VPN only add to their ENUM directories the numbers owned by their members. But even if you are not a customer of one of their members, you can still list your number in an ENUM directory, e164.org. This way, anybody who checks for your number in the directory can route the call over the Internet. Calls made this way don’t need to use SIP trunks, and they can complete in HD voice.
If you happen to have an Asterisk PBX, you can easily provision it to check in a list of ENUM directories before it places a call.