Asterisk as a SIP client

In sip.conf under [general] add a register definition:

Format:
register => user[:secret[:authuser]]@host[:port][/extension]

or
register => fromuser@fromdomain:secret@host

or
register => fromuser@fromdomain:secret:authuser@host:port/extension

Example1:
register => username:pwd:username@216.144.238.155/extension

; Register 2345@mysipprovider.com with authname 2345 at mysipprovider.com at sip provider as 1234 here:
  register => 2345:password@mysipprovider.com/1234

; Register 1235@mysipprovider.com with authname 1235 and realm mysipprovider.com at proxyof.mysipprovider.com as s here.
; (as needed by budgetphone.nl):
  register => 1235@mysipprovider.com:password:1235@proxyof.mysipprovider.com