Method and system for sensing state of user to perform IMS service and give notice

The invention discloses a sensing un-IMS operation state of user and informing method and system, said method includes following steps: a. know the inside story network entity detects the state of un-IP multimedia subsystem IMS grouping operation, and reports it to network region selecting entity NeDS. The invention overcomes defects of current technique, and makes network region selecting entityNeDS in double network operation scene can sense the un-IMS grouping operation state of user for subsequent route control.