What causes an iMessage to be delivered as a text message? Sometimes when I text other iPhone users, messages don't deliver as iMessages and say "Sent as text message". What causes this to happen?
When the receiving end did not set up iMessage on the iPhone, or when there is no internet connection available (for both receiver and sender), it will fall back to plain SMS messages.

You can disable this option in Settings &gt; Messages &gt; Send As SMS.