asterisk users - Sep 2004 - phones and atas timeout..force * restart

It seems that lots of my phones and ATAs loose their connection with * at
wierd times and never both to "re-register" with *. Is there any way
to
force some timeout in * to ask "Are you still there"?

I have 1 SIP Cisco 7960G that does this and a DLink DVG-1120M (MGCP) that
also does it.

How can I get * to keep them alive?

Thanks,
Matthew