is there any known issue for call forwarding or Fail Over destination (extension ) as it suddenly stopped functioning and getting answering machine of "Call can not be completed as dialed"
No, there is no such know issue.
www3 seems to have a peering problem since midnight: https://www2.pbxes.com/nagios/cgi-bin//e...service=PEERING
Try to move you account to www1 in the meantime.
Peering in our monitoring system refers to trunks to German Telecom. This has been fixed.