asterisk users - Jan 2007 - On-hold calls dropped when new call comes in

Hi,

We have a very basic setup of Asterisk 1.2 with a 4 inbound line Digium card.
The phones are
Grandstream GXP-2000 with the latest stable firmware.

When we get calls and put them on hold and then get a new external call coming
in, it drops the
person on hold. They just get disconnected.

What could be wrong?

Note: we have changed the phones firmware a few times but that made no
difference. We are even now
connected to different external telephone lines and it stayed the same. So
somehow I think it is our
Asterisk config.

Regards,
Lars