This pops up all the time on iPhone 6. What’s going on? Is there any charge? How can I fix it?

This is a prompt from Apple's server. iMessage is a tool for sending messages (text messages) between Apple devices through WIFI or 4G. Generally, iMessage needs to be activated before it can be used. Sending international text messages is one way to activate it. Some operators will charge a fee to activate SMS messages, and some operators will not charge.

FaceTime registration text message fee standard

iPhone users need to activate and register in advance to use the FaceTime video phone function. The mobile phone will send an international registration text message to Apple's server (the number is: 447786205094 ) to activate the FaceTime function. This international registered text message is for China Unicom users (including 2G and 3G users).