Audio crackling after DTMF

I run Asterisk on Centos 5.5 under Vmware ESXi 4.1.

The kernel is: 2.6.18-92.1.22.el5vm and it’s booted off with these options: notsc divider=10 nolapic_timer clocksource=pit

Overall Asterisk runs very well but there is one annoying part that I can’t get rid of.
Audio files do a small crackling/blubber when they get interrupted by a DTMF tone. For example, if the audio file says:

“Press 1 for sales”, if I press 1 in the middle of the audio, I will hear: “Press …”<crackling/garble> which lasts maybe not even 1/2 a second. Almost as if it tried to stop the audio but couldn’t do it in time.

The audio files are in wav format and dtmfmode is inband.

What could be causing that small crackling at the end of the audio ?

Running on a VM.

If you are using Cisco phones over SIP connections, they react badly to changes in timestamp, without a properly announced source change; Asterisk doesn’t properly track synchronisation sources on SIP connections.

Why are you using inband?

Forgot to say that I connect to the system for testing using a cell phone which goes through the PSTN.

Asterisk runs the IVR through SIP however.

Which dtmfmode should I use ?

Current best practice is RFC 2833, but both sides must agree.