a user equipment implementing a message recovery method is disclosed , supporting messages of both the circuit switched domain and the packet switched domain . the ue roams in a wireless telecommunications system and performs a call setup procedure . when the ue receives an nas message comprising a domain identifier and a protocol discriminator , the ue determines whether the domain identifier and the protocol discriminator are of the same domain . if the domain identifier and the protocol discriminator are of the same domain , the ue proceeds with processes corresponding to the nas message . if the domain identifier and the protocol discriminator are of different domains , the nas message is determined to be an error message , and the ue modifies the domain identifier of the nas message to generate a recovered nas message , and determines whether the recovered nas message is compatible to the call setup procedure .