SMS – Country code phone number

Hi,

How does the SMS work with country code?
Do I have to get another plugin that will add this to the orders?

I read in another thread that it checks by IP, but I tested this with my own number – If I add country code to the number in checkout it works. If I dont include country code it didnt work.
My store is German and my phone number and IP is in Norway, if that matters for this test.