VoIP Cookbook: Defining SIP Channel in sip.conf
Every SIP client and server is identified with a text block such as the following:
[xxx] type=yyy parameter1=value parameter2=value
Where “xxx” is the name associated with the SIP client, or any arbitrary name used by other configuration file to refer to a SIP device. Typically, a SIP phone with extension number 123 will have an entry that begins with [123]. Please note that you still have to activate extension 123 in extensions.conf file so that people can call to extension 123. The parameter Type must contain "user", "peer" or "friend".
Asterisk will match the incoming call with the name of the device and type=user of the SIP protocol conversation in column From: user name (without acknowledging the SIP domain). Another way also used is to match the SIP request and [xxx] in sip.conf file, through both the IP address of the requester and the peer information of part of [xxx] in sip.conf file. If host=dynamic, it's not possible to perform matching until the SIP client is registered.