Section: 5.5.7 DTMF protocol control procedure
Content: Dual Tone Multi Frequency (DTMF) is an inband one out of four plus one out of four signalling system primarily used from terminal instruments in telecommunication networks. The support of DTMF in the network is described in 3GPP TS 23.014[ Support of Dual Tone Multi-Frequency (DTMF) signalling ]  [12]. The mobile station shall be capable of transmitting DTMF messages as specified in this subclause if and only if the mobile station has the user connection for speech attached and an appropriate channel is available. The transaction identifier used by the DTMF messages shall be that of the attached speech call. NOTE 1: The present document means that DTMF messages can generally be sent in the active state of a call in speech transmission mode or when a traffic channel is available during setup or release and the progress indicator IE has been received. NOTE 2: Since the DTMF protocol messages are sent in a store and forward mode on the signalling channels the control of the device at the far end may be delayed dependent on the load or quality of the channels. NOTE 3: The procedures described in this paragraph support DTMF only in the direction mobile station to network. A mobile station supporting multimedia CAT during the alerting phase of a mobile originated multimedia call establishment should also be capable of transmitting DTMFs during a multimedia call as specified in subclause 5.3.6.5.