I used this back in the day when I had an iPhone 3G and no SMS plan. I used an app that allowed a text like interface for email and then created an apple script

I used this back in the day when I had an iPhone 3G and no SMS plan. I used an app that allowed a text like interface for email and then created an apple script that went through my contacts to look up their carrier and add their gateway address to their contact card. Looked weird to them to see a text from vividsiphone@ymail.com, but it worked!