Param "extension-in-contact" is used to force what the contact info will be in the registration. If you are having a problem with the default registering as gw+gateway_name@ip you can set this to true to use extension@ip. If extension is blank, it will use username@ip.
if you need to insert these the FROM digits to to the Contact URI User Part when sending call to gateway
From: "8885551212" <sip:email@example.com> Contact: <sip:mod_sofia@firstname.lastname@example.org:5060>7080>
try adding these to gateway params
From: "8885551212" <sip:email@example.com> Contact: <sip:firstname.lastname@example.org:5060>7080>
In addition to the parameters you can optionally set variables to set on either incoming or outgoing calls through this gateway. You set a direction, which sets it on both incoming and outgoing calls if omitted.