phones are devices that connect to Asterisk. They register with Asterisk acting as a SIP location server/registrar and use Asterisk as the outbound SIP proxy. They get calls from Asterisk and place calls to Asterisk. The phone use one of the SIP domains that are hosted within your Asterisk server. (this is like the current "friend")

service is when Asterisk is the UA, acting as a phone towards another SIP server - we register with a SIP location server/registrar to get incoming calls. We place calls, masquerading as a phone (using the registrars domain). Currently, this is a mixture between a peer (matched on IP for incoming calls) and a register= statement. In some cases, two peers and a register= statement. Very confusing.

trunk is when we exchange traffic with another server. We send calls to their SIP domain and receive calls to our SIP domain. We may use realm based authentication for the incoming part of the trunk (not based on caller ID/From: header) and a combination of SIP domain and ACLs. This is currently handled by defining sip peers for outbound calls and separate SIP peers for inbound calls - where we match on IP. The problem with the IP matching is when a trunking partner use several SIP servers to connect to us, we need to define one peer per server instead of just matching on domain and then authenticate.

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Chan_sip3: Devices, trunks and services