[plttn]

iMessage doesn't even warrant a dialog in OOBE on iPhone IIRC.

Not sure what pop-up you're talking about then.

[kabwj]

https://macreports.com/wp-content/uploads/2017/03/your-carri...

This shows up when you insert a sim that’s not activated with iMessage. Maybe it depends on your region. In my country you really have to pay because the activation process sends an sms message.

[stiGGG]

I've seen this popup, too. It will send a silent SMS from your device on activation. Depending on your plan this could cost you money.

[eigenvector]

And depending on where you are, it may be an international SMS, which will either cost a lot or just fail silently.