Copied and pasted from a thread on another forum (addressing an identical problem):
"It means that your friend has a phone tracker application and you must have been trying to test it using your number to trace. The best solution is that go to your friend`s phone and deactivate the setting that you made to track your phone. This usually happens when a sim is inserted to that of phone with tracker. It informs you that the sim of the phone has been changed when you receive a text message containing the same content."