Je dois avouer, pas moi (dit le chat...)....

Document dans la racine des sources asterisk, a lire impérativement...

Notamment, on peut faire du "dialplan injection" dans asterisk:

[....]
For example, instead of just sending a request
to dial extension 500 (which in our example above would create the string
SIP/500 and is then used by the Dial() application to place a call), someone
could potentially send a string like "500&SIP/itsp/14165551212".

The string "500&SIP/itsp/14165551212" would then be contained within the
${EXTEN} channel variable, which is then utilized by the Dial() application in
our example, thereby giving you the dialplan line of:

exten => _X.,n,Dial(SIP/500&SIP/itsp/14165551212)

bonne lecture.....