Why is the other party an Apple phone, and I send a text message to show it? I turned on instant messaging, didn't I?

When the iPhone sends a short message, the short message report shows that "sending through the short message service" means sending a short message through the operator's short message service, which means that there is a short message usage fee.

Apple's iMessage short message service is free, but the receiver's mobile phone needs to be enabled with iMessage short message service function, and it must be supported by the network. When both parties have opened the iMessage short message service function, they can't send iMessage short messages due to poor network signal. At this point, if the "Send as SMS" switch is turned on under the information settings, they will automatically send a message that ordinary SMS cannot use iMessage. At this point, the SMS operator will

If the "switch" above is turned off, when the iMessage message cannot be sent, there will be a "Cannot Send" prompt, and the color of the message will also change.

IMessage messages are blue, which is the color set to distinguish them from ordinary messages.