TS2755: iOS: Troubleshooting MessagesLearn about iOS: Troubleshooting Messages
Settings>Messages>Send and Receive at>You can be reached by iMessages at>. If the other phone number appears in that setting, uncheck the current phone number and check the number that you want to use. If the second phone number does not appear at all, you may have to turn off/sign out of Messages on all devices and then turn it on and sign in again.
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