My phone is sending messages through sims instead of iMessages. Why?

I should add to my original reply...

Texts sent between Apple devices can be sent via iMessage when your iMessage option is on.

Texts sent to google devices (a.k.a. androids) will be sent as SMS.

In a message thread, between you and another person, you'll either see the texts in blue or green bubbles (there is one other color possible but it's strictly reserved for business messaging).

Blue text bubbles = text was sent via iMessage to a fellow Apple device user.

Green text bubbles = text went to a non-Apple device user (google supported device)

While these colors are mostly true, there is at least one exception to the rule that I'm aware of. If you're in an area with bad cell service, it's possible that your phone will send a regular SMS to someone that they normally go to as an iMessage. The same thing can happen when the person you are messaging is in a bad cell area. Texts from them might arrive to your device as an SMS instead of an iMessage.

I hope that you find this helpful! 😉

Check this:

Settings, >Messages, iMessage button is "On" ???