This paper evaluated resilience of the reference IMS based network topology in operation through the keys reliability parameters via OPNET. The reliability behaviors of communication within similar and across registered home IMS domains were simulated and compared. Besides, the reliability effects when increasing requested traffics were presented; as well, the results were compared when applying equally load balancing of a 1:1 redundancy of S-CSCF unit into the core registered domain. The results exposed insight reliability behaviors of communication within similar and different registered domains.