Network/Line Unavailable Forwarding

I am sure there is a way to do this with Kamailio/Opensips but can't tell.

Say I have a Openser(Kamailio/Opensips) core that connects to my ITSP SIP trunks and also to my remote PBX's(Asterisk or Freeswitch) via SIP trunks. If the WAN connections go down at the sites with the remote PBX's I would like Openser to know that the specific remote PBX is down and forward any incoming calls to a specified backup number (cellphone, etc) for each individual DID at that site. So if the site has 3 people with 3 different DIDs Openser would know to forward the following
DID 1 is xxx-1001 - forward to cellphone 333-xxxx
DID 2 is xxx-1002 - forward to cellphone 444-xxxx
DID 3 is xxx-1003 - forward to cellphone 555-xxxx

Is there a way to do this? If so what module or modules?

If you have rules that can map to regular expressions, then you can use dialplan module:
http://kamailio.org/docs/modules/1.5.x/dialplan.html

If not, you can use alias_db module:
http://kamailio.org/docs/modules/1.5.x/alias_db.html

Thanks for the reply. I will look at those modules.