Turns out Messages makes decisions about how to send your message based on what it knows about the sender and recipient's connections. Sometimes it will send SMS text messages but it will usually try to route them over the Internet through their own servers from one user to the other. It can be confusing since the same app is used for both message types and Apple doesn't specifically tell you which method it chooses. It does tell you though.
As a general matter, it's a good idea for Messages to try to use the Internet where possible. The cost of messages over the Internet (or at least the marginal cost) is zero. SMS Text messages use SS7 signaling over the phone network and carriers charge for them. It may be that you have an unlimited texting plan so your marginal cost is zero, but you did pay for the plan.
But the opportunities for things to go wrong with messages sent over the Internet are many, and so such messaging is less reliable.
Would you like to change Messages so that it only uses SMS messaging? Here's how:
- Open the Settings app
- Scroll down to the Messages settings entry. it's grouped with other iOS entries like General and iCloud. Touch Messages.
- If the iMessage setting at the top is on, touch it to turn it off. The screen should look like the image below this article.
Now Messages will only use SMS.
Thanks to Neil McGrath of Acision for pointing this out to us.