impvan,
No, there wouldn't need to be any additional cards installed for SIP Trunking as it is all handled by the processor. What Mitel recommends, as a best practice, though is that an MBG [Mitel Border Gateway] be installed to help support SIP traffic and provide better security, but it technically isn't needed as the MiVO-250 can perform without it. The only issue you will run into is how to configure it for a SIP provider that uses SRV records as it isn't apparent on how to do that but it can be done.
If, your system is having a hard time processing the information, which is shouldn't, then there are two options moving forward.
1. Get the PEC-1 [Processor Expansion Card - 1] which may or may not already be installed.
2. Get the PS-1 [Processing Server - 1] which may or may not already be installed.
I have never had to install either of those processing units just for SIP in case you were wondering.
Thanks,
TE