You can not use the Busy Buddy with the AT&T Marathon when going through the AUX jack of the phone.
The size of the TELL WALL JACK cord is an RJ11 and the Busy Buddy connector cord is an RJ9.
If the Customer really wants a Busy Buddy with their partner phone they can use the BASE JACK cord and plug it into the handset port of the telephone. You can then plug the curly cord into the back of the AT&T Base and set it to “lifter mode”
**If the Customer decides to use the BASE JACK cord please let them know they will no longer have ring detection and they will need to take the receiver off the cradle**