Tactical Software Knowledge Base

How do I send DTMF signals through a Cisco digital modem?

Cisco routers with digital modems require that an "X" character be placed in front of any DTMF data (dual tone multi-frequency) that is sent in a dial string after a phone number.  For more information see:

If you needed to dial the phone number 1234567890, pause for 15 seconds and then dial 12345, the dial string would need to be sent to the modems as 1234567890X,,,,,12345

However, this may create another issue.  Microsoft TAPI can strip out characters in a dial string that it doesn't like and in some cases we have found the "X" character can be removed.

For example:  In HyperTerminal, go to File | Properties and uncheck the "Use country/region code and area code" checkbox.  Now when a call is made the "X" character will be sent through.  When this box is checked, HyperTerminal will remove characters (including the "X") that it doesn't consider to be part of the phone number.

Other dialout programs will vary in how they work.  If you find that this solution doesn't work with an application that you need to use, We suggest that you contact Cisco.  They made a patch in the IOS for certain routers that will allow control over what character is used to signal the start of DTMF signals.  Then you can choose a character that TAPI will always pass (such as @).  If you do need to contact Cisco about this, you should refer to case SR 600482576.

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Last Updated
31st of August, 2009

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