asterisk users - Sep 2003 - chan_zap "Cannot handle frames in 2 format"

I have discovered something quirky in our Asterisk.  If I call in to a 
Zap channel (from an outside POTS line), then transfer the call around 
several times, I get the above error, after which it will hangup.  I 
believe Asterisk may issue a SIP CANCEL to the extension it was starting 
to dial.

Now when I say 'transferred around several times,' our routing is pretty
compex and uses the database lookup for user extensions.  It plays a 
static message, then goes to a 'which user do you want' type menu, then 
may go to voicemail or ring an extension, while I beat on it with 
Redirect commands through the management interface.  I sometimes 
redirect it to specific SIP extensions and sometimes to users, which 
have to be looked up in the database.

The SIP phones are set to communicate with Asterisk only using mu-law. 
The IAX connection uses GSM (and we do have multiple Asterisks talking 
over IAX).  One thing that puzzles me is that the "format 2" in
chan_zap
I believe corresponds to GSM.  Where is it getting that?  It should only 
be using mu-law on the local system.

The only other possibility that occurs to me, is that voicemails are 
left in GSM format.  Is it possible that if a call gets transferred 
after it's already in the process of leaving a voicemail that will break 
it?

Suggestions?

Thanks,

Matt