Hi, it was introduced for sure in 8 UR 3. I will be upgrading 10 clustered sites to this level soon and I will need it turned off as well, the PA's are bound to hate it. I assume it will affect secretarial keys the same way. I can't see anything in the class of service options to turn it off, it may turn out to be a command entered at shell level in Vx works maybe.
I'll raise a ticket with Mitel over this and let you know the answer if and when I get one.