DTMF works sometimes

I am running Asterisk with a Rhino R24FXX-EC analog card. Most of the time, the system works just fine, but I occasionally get complaints from heavier users that DTMF is being ignored. The suspicion is that the condition arises after a long pause in dialing but I have yet to confirm this. Apparently receiving a call clears up the condition on the line. Occasionally, pressing a button on the phone until one ‘takes’ is a work-around but that doesn’t always work.

I’ve done some debugging on this using ztmonitor and the tones seems to be coming into the system just fine but they are not being seen by Asterisk. I would think if it were a timing issue on/to/from the card that all lines would see the issue when it occurs but it does seem to be tied to the line. I’ve tried adjusting gain levels on the lines with apparently little effect.

I’d appreciate any suggestions.