By defualt that service is disabled on Nodes, you need to add this line to on the remote nodes you want to control. Be advised why this was disabled was, someone could control your node.
This needs to be added to the rpt.conf
remote_dtmf_allowed = 1
so first on the node that your node is connected to (or chain of one)you want to control you enter *4xxxxx X's is the remote node number, then any DTMF you do will be sent to the Node, but actually does not send DTMF tones, it uses the serial data connection you have between the 2 nodes via connection.