smb@research.att.com (03/21/89)
Correct me if I'm wrong, but I thought that some pay phones disabled the keypad as a side-effect. The intent was a polarity reversal on the line, as I recall; that in turn will have the effect of disabling the keypad on older phones. I don't think there's any attempt to keep you from further dialing (the COCOTs being a notable exception, of course).