There is no SIP card or special interface for SIP trunking in the phone system itself, only licensing.
That said, as VinceWhirlwind and I have both said, a SIP Session Boarder Controller is often required depending on the situation, and the Mitel Border Gateway is a common recommendation (I recommend it as well).
Now, in some situations like my company's hosted SIP trunking service... We provide an Adtran Netvanta router, and it requires a static public IP and an internal IP in the voice VLAN, it *IS* the SBC and we require it due to encryption and security concerns. A separate SIP SBC like a Mitel Border Gateway is not needed in this type of scenario.