I'm suggesting this ONLY because I've done this myself...
Have you looked at the button table for the specific phone(s) in question to be sure the Acct button is actually there?
The reason I say this is because I set up 30 phones in an ACD group one time and after the fact the users decided the CNCT button was more useful that the CONF button to them so I went in and edited the button table thinking that would fix it for everyone at once.
Unfortunately that did not change the phones - just the button table for all future phones, and I had to go back to each phone and reassign the same button table and hit change to make the changes take affect.
In your first message you asked if you needed to put the ACCTCD button on the phones and the response was yes. If you did what I did and just edited the button table for those phones it probably did not actually make the change at the phone level.
If that is the case just go to each phone, reenter the table number, hit change and you might fix your problem.