As far as I know, there is no such thing as a Xerox 6651i.
If it is in fact a 6655i, there is no way to do dialing prefixes, I don't think any current machines have that capability, all you can do is add the common fax numbers in to the address book with the prefixes .
Looks like most of the competition has done away with it too. Seems to me it would have been easier to keep the old fax code than to eliminate bits of it, but I am not an engineer or a developer.