How to avoid iMessages services carrier charges after every restart?
Apple messaging service works with an AppleID and your iPhone mobile number. Since SMS activation is only required if you want to use your phone number with Messages / Facetime chat, removing your phone from  Messages should stop the request for repeated SMS activation.
To remove your phone number from Messages follow these steps:

On your iPhone, go to Settings &gt; Messages &gt; Send &amp; Receive. Select your Apple ID and tap Sign Out.

Then go to Settings &gt; FaceTime, select your Apple ID and tap Sign Out.

Additionally, some may require to go back to Settings &gt; Messages &gt; Send &amp; Receive and  Tap “Use your Apple ID for Messages” and sign in with your Apple ID.


(Note: If Messages and / or Facetime section in settings is disabled / greyed out, go to Settings &gt; Screen Time &gt; Content &amp; Privacy Restrictions &gt; Account Changes and tap 'Allow')
Source: Add or remove your phone number in Messages or FaceTime on all your Apple devices.
ºº(This didn't work for me on an old iPhone with an outdated version of ios and still kept asking for SMS activation. So update ios on your device if you still have issues.)

I consider this an issue with Messages / Facetime design and I would even label it a bug, even if Apple doesn't consider it so.
This issue is even more irritating for users who have to pay international rates for the SMS that Messages / Facetime wants to send to Apple for activation.
This is an Apple issue and not a carrier / cellular phone provider as this is about activation of an Apple service (iMessage / Facetime). So talking to your carrier support won't help with this.
Before you can use Messages / Facetime chatting service on your iPhone, it requires that an SMS be sent from your phone to Apple to activate it. This SMS is sent the first time you try to use Messages / Facetime or if ios thinks your Messages needs to be activated. So for me, and many others outside US, it sends a costly international SMS to Apple.
This SMS activation is required if you want someone to also be able to locate you using your mobile phone number. (Note that by Messages I mean message that is sent through iCloud and internet and not SMS, which are messages sent through your carrier network. You can always send SMS through the Messages app without requiring any such Apple activiation).
I presume that Messages / Facetime keeps asking to send SMS activation on every restart, because it probably wants to verify if you have changed your SIM card or phone number.
There is currently no known method to avoid sending this SMS to activate Messages on the iPhone. (Apple iPad and iPod Touch don't require SMS activation for using iMessage as you can only use your AppleID for Messages on these devices).
The only way to avoid paying the SMS charges is to change your plan to one that includes free SMS (but this option will work only for US citizens, as no plan I've heard of includes free international SMS!). Or to a different carrier that has an understanding with Apple to not charge for such SMS sent to Apple. (Yeah, I know both options suck).

Note: If you frequently have to restart your iPhone because the phone seems to hang or struggle, it may be time to change your iPhone's battery. As your battery becomes older, ios throttles your phone so that the battery is not strained and can give you longer battery life. But ofcourse, this means that your phone becomes slower and everything feels sluggish.
More information on this can be found here -

iPhone Battery and Performance
What is iPhone throttling and why does it happen?