The SIP INFO method can be used by SIP network elements to transmit digits out-of-band as telephone-events in a reliable manner independent of the media stream.
The SIP INFO method contains a message body in addition to the header, that carries the signaling information and uses the content-type application/dtmf-relay
Example
INFO sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP alice.uk.example.com:5060 From: <sip:[email protected]>;tag=d3f423d To: <sip:[email protected]>;tag=8942 Call-ID: 312352@myphone CSeq: 5 INFO Content-Length: 24 Content-Type: application/dtmf-relay Signal=5 Duration=160
In this example the digit ‘5’ is transmitted with an indicated duration of 160 milliseconds.
Another, less common, variation of sending digits with the 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.
INFO sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP alice.uk.example.com:5060 From: <sip:[email protected]>;tag=d3f423d To: <sip:[email protected]>;tag=8942 Call-ID: 312352@myphone CSeq: 5 INFO Content-Length: 1 Content-Type: application/dtmf
Note: dtmf-relay or dtmf are not IANA registered application mime types.
Cisco uses SIP INFO for DTMF relay. Cisco equipment uses the following feature restrictions:
- Minimum signal duration is 100 milliseconds (ms). If a request is received with a duration of 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.
December 2, 2021
Phone System 