Apple's proprietary iMessage system lets iPhone users send text messages to other iPhone users over a data network, avoiding SMS charges and making texting free, at least within Apple's ecosystem. It's an impressive run-around of the entrenched carrier system - the same basic idea, applied to an agnostic model, has made texting alternatives like WhatsApp fantastically popular. But users found that trying to leave Apple's walled garden was much harder after setting up iMessage with their personal phone numbers.