I do use carrieroute to balance calls to 3 servers and those 3 servers do use a2billing and they forward calls to my pstn gw. I do have a graph that has the active calls on them randomly and it does not depend on the load at hand the calls spike up on all 3 servers together. And most of those calls are in ringing state but with no callerID.

ACC: transaction answered: timestamp=1236087551;method=INVITE;from_tag=rMciw2o;to_tag=as7761f5cf;call_id=c745983c;code=200;reason=OK
ACC: request acknowledged: timestamp=1236087551;method=ACK;from_tag=rMciw2o;to_tag=as7761f5cf;call_id=c745983c;code=200;reason=OK
Mar 3 13:39:11 [4286] INFO:carrierroute:cr_do_route: uri xxxxxxxxxxxxx was rewritten to sip:xxxxxxxxxxxxxxx

ACC: transaction answered: timestamp=1236087552;method=INVITE;from_tag=yJ+hSWA;to_tag=as26649b96;call_id=a1c42904;code=183;reason=Session Progress
ACC: transaction answered: timestamp=1236087552;method=INVITE;from_tag=CAJpAXg;to_tag=as5d24ff4f;call_id=a35563d5;code=183;reason=Session Progress
ACC: transaction answered: timestamp=1236087552;method=INVITE;from_tag=7i2kutG;to_tag=as6c125d0e;call_id=2739b561;code=183;reason=Session Progress
Mar 3 13:39:12 [4285] INFO:carrierroute:cr_do_route: uri xxxxxxxxxxxxxx was rewritten to sip:xxxxxxxxxxxxxxxxxx
ACC: transaction answered: timestamp=1236087553;method=INVITE;from_tag=tMs9y8M;to_tag=as18b7f087;call_id=7d65d34b;code=183;reason=Session Progress

However when this problem happens the log screen only has the something like the below and as said the calls are in ringing state with no caller ID:

Mar 3 13:39:11 [4286] INFO:carrierroute:cr_do_route: uri xxxxxxxxxxxxx was rewritten to sip:xxxxxxxxxxxxxxx

Mar 3 13:39:12 [4287] INFO:carrierroute:cr_do_route: uri xxxxxxxxxxxx was rewritten to sip:xxxxxxxxxxxxxx