Sorry, don't have my manuals handy, but just popped the cover off a DK40i and it has a K5RCU card in the bottom of the cabinet which is required to receive DTMF tones from the single line ports - you also need to mark it as installed in system programming - (?) Prog. 03 slot 00 (?)

(?) - Verify that in a manual, I'm pretty sure about the Prog. 03, less positive about the slot number, and you still need to know what data to put in that entry.