This should be a red flag to all customers
Essentially Mitel already contained an accumulation of products which just don't work very well. They speak very peculiar dialects of SIP which are sometimes just plain wrong. Adding to that it seems as if you can only get those installed by "certified" "technicians" which typically means that you get a salesperson with nothing more than a in depth knowledge of the marketing material.
Now add the typical business model of a hedge fund to it, and you'll get a company essentially stopping all bug fixing to change marketing related things. The code probably will be bad enough that even changing "Polycom" to "Mitel" in the strings will change the memory enough layout so the buggy code won't work any more.