Automatic sms sending.

If your iPhone is used with a carrier not found in the wireless carrier articles, an international SMS is sent each time you activate iMessage/FaceTime. (One SMS Only)

If there are 4 SMSs that indicates you have activated iMessage 4 times.

Each time you change the SIM or activate iMessage/iCloud/FaceTime you must have got these warnings (As Shown in the pic below). If you had tapped on OK then only then the SMS is sent .....

See the pic below

If you wish not to incur international charges then please do not activate iMessage for phone number, as shown in the pic below.

This is one time charge for activation by your carrier. Your iMessage would have been free for life time. Now that you have de-activated, it will be charged again when activate by your carrier. Sorry, you de-activated it without understanding it.

Read this Apple Article: If you can’t turn on or sign in to iMessage or FaceTime - Apple Support (IN)

An extract from the above article of it is given belo....

"Check your device settings

1. Make sure that you’re connected to a cellular data or Wi-Fi network. If you're using an iPhone, you need SMS messaging to activate your phone number with iMessage and FaceTime. Depending on your carrier, you might be charged for this SMS."

Do you receiving the message when you are trying to activate iMessage and Facetime?

See this Apple article ☞ if you get an error when trying to activate iMessage or FaceTime.

Perhaps Is your Wi-Fi network Is unavailable?

Turn on Wi-Fi network: Settings ☞ tap on Wi-Fi ☞ turn Wi-Fi ON

You have to make the Wi-Fi network available, because the phone sent 'silent' message at the background to the server, trying to activate your request.

Then you get charged.

If you are using an unsupported carrier, and you will be charged for the background text messages used to activate FaceTime and iMessage.

It is your choice to activate knowing you may be charged, or to cancel and not use these features.

Or you switch to a supported mobile carrier.

In order to register your iMessages & Facetime apps, yes, your iPhone must send an SMS internationally.

But this happens just once (when your iPhone is activated for those apps, it doesn't need to send any other international SMS).

See point #2 at the section "Check your device settings" in this support article:--> If you can’t turn on or sign in to iMessage or FaceTime - Apple Support

Regards

Giulio