The whole idea of this cookbook is to make HD Voice phone calls. But as described in Step 7, the way that virtually all phone calls are addressed is with regular phone numbers, called E.164 numbers. The problem with this kind of addressing is that the easiest way to complete calls is via the PSTN, which always busts a call down to a narrowband codec – PSTN gateways usually negotiate G.729, which is marginally poorer in quality than G.711 which is how calls travel once they are on the PSTN.
The way to avoid passing through the PSTN is to address calls with SIP addresses as described in Step 7, rather than E.164 addresses. But what if you want to call someone but you only have their phone number, not their SIP address? There is a technique called ENUM (E.164 NUmber Mapping) that uses a database to translate from E.164 to SIP addresses. Actually there are several ENUM databases. ENUM is patchily deployed; some countries have a strong ENUM implementation, others not. The statuses of the various countries’ ENUM deployments are documented here.
I added the number of my POTS (Plain Old Telephone Service) line and associated it with the address of my IP 650 phone. That way when people dial that number from a regular phone it rings on my POTS line, but when someone calls that same number from an IP phone it rings on my IP phone. In theory. Actually, not all VoIP providers do the ENUM lookup, because it adds a few seconds to the call setup time. Gizmo5 does (it calls it Backdoor Dialing), OnSIP doesn’t.