The Mitel 3300 requires a SBC of some kind for SIP trunking in most cases... Either a Mitel Border Gateway or other SBC like Adtran or AudoCodes, since the Mitel does not support NAT in any way.
Programming is a little more complex and has some gotchas in the Mitel, even in the basic trunking, like your Trunk Attributes must have a Dial In Trunks Incoming Digit Modification - Absorb defined, even if that is 0. There are plenty of "general" SIP trunking guides in the Mitel knowledge base, have you gone through any of those?