asterisk users - Jan 2006 - Incoming calls grind to a halt

Hi there everybody,
	We are running Asterisk 1.2.1 with a TE410P card attached to one
PRI ISDN line, and many SIP phones. Yesterday we ended up in a situation
where all incoming calls were giving the engaged tone. Every time some
tried to ring in we got:

Jan  4 14:56:32 WARNING[896] chan_zap.c: Ring requested on channel 0/5
already in use on span 4.  Hanging up owner.

This happens even though no calls were being made or received. This
happens semi-regularly after use of the server for a while. A reboot
solves the problem.

Dialling out works fine.

I appreciate I haven't given enough information- I've hesitated from
attaching the log file due to it's huge size. I'm not sure which bits
are pertinent.

Can anyone offer any advice?

Thanks,
	David