Please explain how this number can be incorrect ? When the sms is sent, it uses the sim inserted at the moment in the phone, and thus the telephone number associated with this sim. The phone number shown is handled by the carrier network, not theft aware.
However this doesn't mean that the sim which was used to send the sms is still present in the phone. For instance a stealer can remove the original sim card, place a new one to check if the phone is working ->trigger the sim notification, then remove the sim card. When you receive the sms you'll have the number of the sim card used by the stealer but it will be useless because the sim was removed from the phone.
