asterisk users - Feb 2007 - chan_zap.c:3926 zt_handle_event: Ring/Off-hook in strange state 6 on channel 1

I sometimes receive this error with an incoming ring:
chan_zap.c:3926 zt_handle_event: Ring/Off-hook in strange state 6 on channel 
1

The symptom is: when I pick up the extension (Console or SIP), ringing 
continues.

Conversely, when I make a Zap call from SIP or Console, the recipient and 
caller can talk, but Asterisk continues to show the state as Ring. (Normally 
the state is "Up".)

I am not totally sure about how to reliably reproduce this, nor how to 
correct this without reboot.  I can stop Asterisk then rerun ztcfg.  
Sometimes this fixes the problem.

Hardware is X100P clone (SM56), zaptel 1.2.10, Asterisk 1.2.13.  Anyone else 
seeing this?

Yuan Liu