I did the same thing with a CTX-28, because it was not sold with a GETS. Then someone told me that the 28 comes with a built in modem, and 1 analog port. You can dial into the system from the analog port with modem(#19).
That was after spending all day with the manual, and not being able to program some features because I only had a 10 button phone.

For a while there I could not find the phone programmers guide on FYI. I do a few minor changes by phone if I don't already have the laptop booted.