The above commenter explained how. iPhones will sometimes simply refuse to send an actual text message if the phone number was once associated with an iMessage account. I have deregistered my phone number from iMessage, iCloud, and whatever else, and my parents' iPhones still send me iMessages despite texting my phone number, even replying to a message or geoup that I text messaged from my phone number. Apple has already been sued about this before. That and picture and video sending is horrible, I think because Apple has historically not implemented protocols that Android exposes.