You can (sort of) do this:
Toll restrict the extension.
Enter the allowed-to-call number in an AllowList table.
Assign the extension to that AllowTo List.
Program a button on the phone to AutoDial the number.
Note that you cannot enter the entire phone number in an AllowList, can only enter a "1" plus six digits. So it is possible for the phone to call another number within the same area code and exchange. The AllowList will not look at the last four digits of an eleven-digit toll number, so a button for auto-dialing would be a good idea and hopefully the user will not figure out they can manually dial a different, similar, number.