Port Farwarding & Nat issue

Hi,
We are using Elastix 2.0.0-63 and also latest version at different location. Both servers are not inter linked(different)
using this server since 2 years kept in Nating, port forwaded from firewall to the Public IP. Till last week it worked perfect since last week we are unable to make out bound suddenly getting message that all circuits are busy not connecting to my sip provider means 3rd server VOS 3000.
after restart the server one or more times,starts working , again within 24 hrs it is same getting same message.
We have not changed any settings in firewall in recent days.
Both servers are in different users location but problem is same
Can you suggest the reason to resolve.
Thanks in advance