gnu wrote:Of course. If the phone isn't registred, how can the phone know how is his proxy server?
Of course, the phone have to register first!
If the phone doesn't know the identity of the proxy, in practice it won't know the address of the registrar!
The real answer is more complex.
The phone doesn't need to register in order to make outgoing calls.
In principle it needs to register in order to receive incoming calls.
The former is complicated because some phones and some servers may not play fair in this case (e.g. you may find you need to provide a full SIP URL, not just a phone-number like user name).
The latter is complicated by workarounds for commercially imposed dynamic IP restrictions. If the entity making the last hop towards the phone (which could be another phone, not just a PABX or proxy), has a domain name, registration should not be necessary. This is subject to caching policies at the last hop originating end, and where ISPs deliberately change addresses on live connections, to the response time of any dynamic DNS server update.
I believe that Asterisk caches DNS lookups when it parses sip.conf, so I think that specifying host= with a dynamic DNS name will fail if the address changes after the last sip.conf reload.