The SIP INFO method can be used by SIP network elements to transmit DTMF tones out-of-band as telephone-events in a reliable manner independent of the media stream.

In the DTMF relay method the body of the SIP message consists of signaling information and uses the content-type application/dtmf-relay

Example


In this example the digit '5' is transmitted with an indicated duration of 160 milliseconds.

Another. less common, variation of sending DTMF with INFO method is the DTMF trigger mechanism which uses the application/dtmf mime-type. The body of the message consists only of the DTMF digit.


Note: dtmf-relay or dtmf are not yet IANA registered application mime types

Cisco uses SIP INFO for DTMF relay: See http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t11/ftinfo.htm
Cisco equipment uses the following feature restrictions:
-Minimum signal duration is 100 milliseconds (ms). If a request is received with a duration less than 100ms, the minimum duration of 100 ms is used by default.
-Maximum signal duration is 5000 ms. If a request is received with a duration longer than 5000 ms, the maximum duration of 5000 ms is used by default.
-If no duration parameter is included in a request, the gateway defaults to a signal duration of 250 ms.

• SIP method info
• SIP DTMF Signalling