The mobile phone received a short message showing iMessage. Who sent the blue message?

The green one is normal SMS, and the operator will charge SMS fee. The blue one is iMessage, which can only be used between iPhone and costs nothing. The premise is that the mobile phone turns on iMessage and the network, because iMessage uses traffic.

IMessage can be opened as follows:

(1) Select "Settings" on the phone desktop, as shown below;

(2) Select "Information" on the setting page;

(3) You can see iMessage on the information page, turn on the switch, and log in to Apple ID to use iMessage (cellular data must be turned on).