The man-in-the-middle proxy adds the phone number based on your IMEI. You could probably send someone else's number down and it'd change the header back to you

The man-in-the-middle proxy adds the phone number based on your IMEI. You could probably send someone else's number down and it'd change the header back to yours in the proxy. The SMS spoofing issue (which has been around forever) is far worse as it's not proxied anywhere. Some SMS-controlled devices use the SMS from number as an authentication scheme which is much much more dangerous as that can be spoofed easily. It's all about trust and it appears that trusting proprietary networks is an issue.