MTP is required in the following scenario

<note important>CUCM software MTP only support G.711</note>

MTP is inserted when point A support only in band DTMF and port B support only out of band DTMF.

Because we need to extract the DTMF from the voice path and put it into the signaling path? It seems even if we are using different OOB methods, CCM can do the conversion without using an MTP. For instance between NTEs and SCCP, because NTE are part of the media stream; therefore Unified CM does not receive them. An MTP is invoked into the media stream and has one call leg that uses SCCP, and the second call leg uses NTEs.

Named Telephony Events (RFC 2833)

• RTP-NTE
• DTMF packets are sent with an RTP payload type that identifies them as NTEs.

Key Press Markup Language (RFC 4730)

• SIP Only
• KPML procedures use a SIP SUBSCRIBE message to register for DTMF digits. The digits themselves are delivered in NOTIFY messages containing an XML encoded body.

Unsolicited Notify (UN)

• SIP Only
• Using unsolicited SIP NOTIFY messages.

H.245 Signal, H.245 Alphanumeric

• H.323 Only
• H.245 utilizes the signaling channeland, hence, provides an out-of-band (OOB) way to send DTMF digits.
• The Signal method carries more information about the DTMF event (such as its actual duration) than does Alphanumeric.

Skinny Client Control Protocol (SCCP)

• SCCP defines out-of-band messages that transport DTMF digits between Unified CM and the controlled device.

Early Offer

• Outgoing Early offer.
  • Disabled by default
  • Enabled by checking MTP required on SIP trunk
• Incoming Early Offer does not require an MTP.

H.323 Supplementary Service

• MTPs can be used to extend supplementary services to H.323 endpoints that do not support the H.323v2 OpenLogicalChannel and CloseLogicalChannel request features of the Empty Capabilities Set (ECS).

H.323 Outbound Fast Connect

• Outbound calls on an H.323 trunk do require an MTP when fastStart is enabled.
• For inbound fastStart, no MTP is required.