Section: 5.5.7.3 Stop DTMF request by the mobile station
Content: When the user indicates that the DTMF sending should cease e.g. by releasing the key the mobile station will send a STOP DTMF message to the network. On sending a STOP DTMF message the MS shall start timer T337. The MS shall only send a STOP DTMF message if a START DTMF ACKNOWLEDGE message has been received from the network (see subclause 5.5.7.2). If timer T337 expires, the MS shall terminate the ongoing DTMF procedure without any retransmissions, and is free to begin another DTMF procedure. (e.g. another START DTMF message).