Can anyone direct me to where I can find any documentation on using QSIG to integrate with 3rd party PBX's?

I am particularly interested in Nortel and Mitel.

You might try the Nortel and Mitel. Tadiran pretty much wrote the book on QSIG and are really the standard. Cisco also has some white papers on it.

Hi, Octel under the guise of Avaya wrote a good and clear guide for integrating their Unified Messenger with a Coral over Qsig and Pri.

https://support.avaya.com/japple/cs...eleaseID=148183&temp.bucketID=108026

Try the link it might be of some use