Just for anyone else reading along now, or in the future, the document that was being referenced is quite old on the Mitel side (release 9 I think), and will only get you part of the way to having things working.
After a few dozen e-mails jf1221 is now all good and able to make calls both ways!
There are a lot of options in the SIP peer profile that need to be changed in order for things to work.
Things like disabling mitel proprietary SDP, and preventing the use of 0.0.0.0, and making sure that early offer is used, are the main ones that will get things going that aren't covered in the referenced document.
You also need to make sure that in the trunk attributes you MUST have a 0 in the DID digits absorb, as without it you get a 503 service unavailable error when you try to make a call from CUCM to the Mitel.
If I get a chance I might try and create a new document as a reference.