If opinions are mixed, it may be clue that this is not a slam-dunk, that there are "considerations" or limitations to the expansion.
What is the real solution? Put the possible limitions of the solution in writing, even a simple fax or email. It doesn't have to be formal or argumentative, just a helpful reminder (which everyone realizes will nonetheless be "on the record.")
The thrust of this note, though, is to alert you to the pitfall awaiting a tech who installs something at the behest of his customer though the tech knows it will be a less than perfect solution. When he does this, he of course will talk with the his contact person who insists nonetheless that the work proceed (because the boss is on their back for a solution).
The problem is that the contact person is not--often--the decision maker or the one who writes the check. In fact, they may be charged by their boss to find the solution and they are on the hook to provide that solution.
So they cajole the tech into installing something that may not work but they report to their boss that a solution has been found. When the doo-doo hits the fan, your contact person has amnesia--they have forgotten or conveniently misunderstood your caveats. They suggest to their boss that you did not do your job or that they were misled.
I have had contact people--who were embarassed and under the gun, to be sure--stand silently next to their boss asked me why something they purchased did not work. The contact person was simply too embarassed to admit to their boss that they were told the "solution" might not fully satisfy them but they chose it anyway. They let the boss think it is your fault. They are just being human, but the heat falls on you.
You cannot win this one: if you contradict your contact person, then they or the boss--or both--will not believe you. And if you don't contradict them, then you are incompetant or deceptive or both.
But your CYA fax or email can go a long way towards an honest understanding: "Gee, Mr. Honcho, I am so sorry this solution seems not to work for you. However, we did know that it might not work because the Endeavor/ACS combo is not really a suppoorted Avaya design option, as I clearly said in my fax to you on such-an-such a date.
Yours,
Mike