I wanted to share my experience on this oldish post as it was the most useful that came up when searching for "BT MAC not accepted".
I am with BE / Sky and asked for my MAC earlier today and was given it verbally - first time incorrectly (an 'I' mistaken for a '1'), second time 'correctly' - but the automated BT system wouldn't accept it. So I tried calling BT with it, who I guess have the same system as the automated one, and it still wouldn't work. I was left wondering what on Earth I was going to do, since BT would probably keep declining it and BE/Sky wouldn't be able to give me any other information than appeared on their screens. So I thought I'd wait for my automated email from BE / Sky with the MAC and take it from there.
An hour later the email arrives. The first part of the MAC is the same as I was given. The second part after the /, that is the AB12C part, was different. Weird. I tried the 'new' MAC. Accepted by BT.
Something must be up with the information being shown on CS screens at Sky and what the system actually emails out. Indeed it may be the case that operators shouldn't give out the MAC verbally and should ask customers to wait for the email, as this is perhaps the 'official' information. Or some other variant of this.
In any case, my experience tells me that it's worth waiting for the official email as what you'll get told over the phone may not be accurate. In my case, that's the story. Hope this helps someone.