Hello!
The Messages app on the iPhone is capable of sending and receiving regular SMS/MMS text messages as well as iMessages. When you send an iMessage, Apple will route that message through their servers using an Internet connection. This Internet connection can be either Wi-Fi or your cellular provider’s data network. If no Internet connection is available, the Messages app may attempt to deliver the iMessage as a regular SMS text message. Depending on your text messaging plan, this could mean texting charges will be billed by your cell phone provider. Also, the person's (to whom your message is being sent) phone might also have this issue, or their device could be off.
I hope I helped. If you have any questions, please feel free to reply to ask me! 🙂
Sincerely,
Westen 😀