asterisk users - Aug 2004 - Intriguing * problem with voicemail signalling

Has anyone seen the following problem?

Until recently, I couldn't understand why some extensions on my * system
would have a "congestion tone" as soon as I picked up the handset.

A little sleuthing through the logs and the source code led me to understand
that * thought it had seen the extension go off-hook, send some DTMF tones,
and then wait.  * treated this situation as a timeout, and switched on
the congestion tone.  For some reason, * never saw the extension go back
on-hook, so the only way to clear the problem (other than a * restart),
was to physically take the extension off-hook and then back on-hook.

A little more thinking and poking, and I think I know what's happening,
but not exactly why:

All three extensions where this happens are phones with voicemail waiting
indicators.  So, I believe the phones go off-hook for a short period of time
after each incoming call to determine if there is now voicemail waiting.

I had thought that they used the stutter tone provided by * to make this
determination.  However, in at least two cases, and probably all three,
the phones can use either stutter tones or FSK signalling to make this
determination.

It appears that, by default, when these phones are first powered-on, they
use FSK, and switch to stutter tone only when that "fails".

My theory is that this attempt at FSK signalling by the extension is
confusing *.  The TDM400P interfaces, which I use for the extensions, must
think that the FSK signalling is DMTF signalling, and misinterprets the
signalling attempt.  It then goes downhill from there.

Any reports of such behavior with * and the TDM400P interfaces?  If so,
is this likely to be a software issue or a hardware problem specific to
the TDM400P?

The only hope I have this there might be a software solution is if the
phone doesn't actually go off-hook for the FSK signalling--then I believe
I could change zaptel.c to ignore any DTMF tones that occur when the phone
is on-hook (if it doesn't already do so).

Finally, any ideas how to solve this problem (other than finding phones
that only use stutter tone voicemail signalling)?

Thanks, in advance.

- Rich