DTMF tones are the tones that are generated when a telephone key is pressed on a touchtone phone. Sometimes the called endpoint needs to hear those tones, such as when you enter digits during the call in response to a menu. Low-bandwidth codecs can distort the sound, however. DTMF relay allows that tone information to be reliably passed from one endpoint to the other. By default, SIP uses in-band signaling, sending the DTMF information in the voice stream. However, you can configure it to use RTP-NTE, SIP INFO messages, SIP NOTIFY messages, or KPML for transmitting DTMF tone information.

RTP-NTE is an in-band DTMF relay method, which uses RTP Named Telephony Event (NTE) packets to carry DTMF information instead of voice.
If RTP-NTE is configured, SDP is used to negotiate the payload type value for NTE packets and the events that will be sent using NTE.

RTP-NTE can cause problems communicating with SCCP phones, which use only out-of-band DTMF relay. In a CallManager 4.x network with SCCP phones, you must provision an MTP for calls that traverse the SIP trunk. This MTP translates between in-band and out-of-band DTMF signals. You must configure a separate MTP for each side of the SIP trunk. You can do this MTP in hardware, or in software on CallManager.

Cisco has two out-of-band procedures for DTMF relay. One uses SIP INFO methods, and the other uses SIP NOTIFY methods.
The SIP INFO method sends DTMF digits in INFO messages. It is always enabled. When a gateway receives an INFO message containing DTMF relay information, it sends the corresponding tone.

NOTIFY-based out-of-band DTMF relay is negotiated by including a Call-Info field in the SIP INVITE and response messages. This field indicates an ability to use NOTIFY for DTMF tones and the duration of each tone in milliseconds. Using this method can help SIP gateways interoperate with Skinny phones. You can also use it for analog phones that are connected to Foreign Exchange Station (FXS) ports on the gateway.

When a DTMF tone is generated, the gateway sends a NOTIFY message to the terminating gateway. When that gateways receives the NOTIFY, it responds with SIP 200 OK and plays the DTMF tone. To configure the DTMF relay type, use the dtmf-relay command in dial-peer configuration mode. To optionally configure the interval between NOTIFY messages for a single DTMF event, use the notify telephone-event max-duration milliseconds command in SIP UA configuration mode. The default is 2000 msec; the lowest value between two SIP peers is the one chosen.

KPML is another way for SIP phones to send dialed-digit information. Like SIP-NOTIFY, KPML uses a NOTIFY message to transmit each digit.
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