Text vs iMessage
Why is my iPhone sending text messages instead of iMessage to other iPhones
Why is my iPhone sending text messages instead of iMessage to other iPhones
If the iPhones you send messages to have not activated iMessages, you will send normal text messages.
You can also check yourself whether iMessages are activated on your iPhone:
Settings → Messages → enable iMessages
You can find more useful information here:
If you can't send or receive messages on your iPhone or iPad - Apple Support
If the iPhones you send messages to have not activated iMessages, you will send normal text messages.
You can also check yourself whether iMessages are activated on your iPhone:
Settings → Messages → enable iMessages
You can find more useful information here:
If you can't send or receive messages on your iPhone or iPad - Apple Support
Text vs iMessage