DTMF - Inband

I’ve read just about everything I could find on the subject, and must have missed something important somewhere.

Here’s my situation. I’ve got a mostly functioning FreePBX (2.5.1) install, Asterisk version (

I have a SIP trunk built to a softswitch that exists in my local network (no firewall.) The softswitch only supports inband dtmf, do to a hardware limitation.

I can make outgoing and incoming calls with no problems all. The problem is, when I hit a voice mail box, IVR, or conference, the system does not recognize any dtmf tones at all.

I placed a call to a softphone and verified that I hear the tones when I press them from an outside line (a call originating on the SIP trunk, terminating to the SIP phone.)

Here’s my incoming trunk setup:


Anyone have any suggestions what to try? I ran across a post at one point talking about adjusting the timing that * uses for the length of the tone, but I can’t find that again, and am starting to wonder if I imagined it.

711 is the only protocol supported by the softswitch. The system does recognize dtmf on calls originated by the softphone directly into *, with the extension configured for both inband and rfc2833.

Is it possible that the issue is the length of the tone, or some other type of adjustment like that?


I found the solution.

I had to enter the following in sip_custom.conf