[Asterisk-Users] using asterisk with iptel addreses

[Graham Turner]

was wondering if anyone could give us a run through an explanation of the
wiki and other examples of connecting to iptel's sip express router using
asterisk pbx so i can understand better the call processing  ..

given the example i work from on john todd's www.loligo.com site ;

exten => _3.,1,SetCallerID(${IPTELUSERID})
exten => _3.,2,SetCIDname(${IPTELUSERNAME})
exten => _3.,3,Dial(SIP/${EXTEN:1}@iptel)
exten => _3.,4,Playback(invalid)
exten => _3.,5,Hangup

are the first two statements mandatory to connection to iptel.org ??

i understand the dial plan of any numbers prefixed by 3 are interpreted as
iptel extensions, and that the third extension priority strips off the
prefix

presumably the call is then 'processed' by the [iptel] section of sip.conf
and generates sip call with the credentials in this section as those passed
to the iptel.org server ??

how (iassuming they do) these relate in any way to the IPTELUSERID /
IPTELUSERNAME variables defined in extensions.conf ??

presumably (and i am using a 7940 sip device) i can dial an asterisk
extension of targetiptelusername prefixed with 3 to call the sip user
registered with

qu. does the target iptel username need the iptel.org domain appended to it
or is it somehow 'implied' by the above ??

another wiki example suggests the use of a 'fromdomain'  statement in an
[iptel] section of sip.conf  ??

TIA

GT