Tanguy,
setting the auto_to_user should set the destination number
field to the correct one according to the wiki
"Note:
extension parameter influence the contents of
channel variable
Caller-Destination-Number and
destination_number.
If it is blank,
Caller-Destination-Number will
always be set to gateway's username. If it has a value,
Caller-Destination-Number
will always be set to this value. If it has value
auto_to_user,
Caller-Destination-Number will be populated with
value
${sip_to_user} which means the real dialed
number in case of an inbound call."