Long DTMF tone duration on outbound PSTN calls

Hello,

We have problems with dtmf on outbound pstn calls: the duration of the generated dtmf tones is inconsistent, generally way too long (2-3 seconds or more). Sending new dtmf digit while the previous one is still playing changes the tone frequencies; any voice input from the SIP client (speaking into or simply touching the mic) ends the tone immediately, and the first dtmf digit that follows is perfectly short… but only the first one.

We use Asterisk as voip gateway sip -> pstn on Dell ML370 G2 running Slackware 12.3 / Asterisk 1.4.26 / DAHDI Linux 2.2.0.1 / Wildcard TE410P (3rd Gen). Same issues with Asterisk 1.4.22.1 and Asterisk 1.6.1.0

The sip clients are configured to send sip info (same problem using rfc2238).
Same behaviour on calls originated at the console:

Below is the Asterisk log showing (1) the sip info events with duration 80 and (2) the long interval between Started / Ending VLDTMF entries. On this particular call the SIP client sent dtfm ‘1’, ‘2’ and ‘3’ with pauses in between, and the digit ‘3’ played for 4 seconds…

Any help/suggestions are appeciated. Thanks, Ivo.