Handling reachability of mobile device when serving core network node changes

A method of handling reachability of a mobile device in a wireless communication system, the method comprising:

• the mobile device establishing a packet data network (PDN) emergency connection when a first back-off timer received from a source core network node is running;
• the mobile device entering a first tracking area from a second tracking area, wherein the second tracking area is associated with the source core network node, and the first tracking area is associated with a target core network node and not in a tracking area list of the mobile device;
• the mobile device sending a first tracking area update (TAU) request message to the target core network node in response to entering the first tracking area;
• and
• the mobile device stopping the first back-off timer if the first back-off timer is still running and sending a second TAU request message to the target core network node, when the emergency PDN connection is disconnected.