Hi Will1981,
It will depend on the other IP-PABX you are connecting the Mitel controller to.
The best thing will be to look at the Mitel Knowledge base to see if Mitel have completed any interop testing, if they have they may have a configuration document to help you program the Mitel system and other PABX.
Even with documentation from Mitel, you may need to make modifications as not all systems perform how you would expect.
Is it a Mitel 3300 (MiVoice Business) or a Mitel 5000 (MiVoice Office 250) that you have?
Andy