Most phones are addressed with a phone number – technically called an E.164 number after the ITU specification that defines the world-wide PSTN (Public Switched Telephone Network) numbering scheme. The problem with these numbers is that the network tends to assume that calls made using them are destined for the PSTN, and consequently encodes them using a narrowband codec like G.729 or G.711. So if you want a wideband call you must use the SIP addressing scheme. A SIP address looks exactly like an email address: email@example.com. If you are using a hosted VoIP service like OnSIP, the domain for your SIP server would normally be that onsip.com, or whatever. But it is more user-friendly to use the same address for both email and voice calls. If you have your own domain name, you can do this by adding records to the DNS (Domain Name Service) for your domain. The DNS records that specify where your emails go are called MX records; the DNS records that specify where your voice calls go are called SRV records. For example, DNS for stanford.cc points to Google for email, to OnSIP for SIP and to www.wirevolution.com for www. You can investigate DNS records for any Internet address here.
There is an excellent explanation of this concept at OnSIP.com, together with links for how to add SRV records to your DNS at GoDaddy.com, Register.com and No-IP.com.
The DNS server for my domain name, stanford.cc, is HostGator, and my VoIP service provider is OnSIP.com, so these instructions explain how to do it for this combination of DNS and SIP servers.
First, you must configure your DNS to forward SIP requests for your domain name to your SIP server. For the type of account I have there, HostGator does not let you administer your own DNS. So I sent the following email to firstname.lastname@example.org:
Please could you add the following DNS SRV record to stanford.cc:
Service: SIPProtocol: UDP
TTL: 1 hour
HostGator customer support modified the DNS records within a day, but didn’t get it working until the second attempt.
- Second, you must configure your SIP server to service requests addressed to your domain name. The OnSIP control panel allows you to specify the domain name for your SIP account for yourself. There are detailed instructions on the OnSIP website.
Before I got my IP phone working, SIP calls addressed to my email address would come to my POTS phone. In other words, somebody with an IP phone could dial my email address and my regular PSTN desk phone would ring. I achieved this by first following the instructions here in Step 7, then, since I didn’t have an IP phone, I configured the OnSIP control panel to forward all calls to my “extension” to my PSTN phone number. It worked perfectly.
There is a good explanation of how to do this for a large organization in the SIP.EDU cookbook.