Today i called customer service. They tried to help me, but in the end it didn’t work. -- First: I can’t receive SMS from international numbers (Europe). And i can’t send SMS to those international numbers. After a few minutes there always is an error message. As i have „$50 Smartphone Plan UNL Talk, TXT & 5GB of Data“ Prepaid, this should work. — And probably this problem is linked to another one: I _never_ see „1x“ or „3G“ in my display. It always says „LTE“. Usually that’s great. But i think, somehow my device isn’t able to log on to Verizons older networks. I don’t have an idea what the problem is. Perhaps one more information: One year ago i tried to activate the same unlocked iPhone 6S with Pageplus. ESN-check said ‚yes‘ - but the device never worked properly on the network. I hope, it's better now with Verizon.
Update: The SMS problem is an iphone problem. International SMS go through by completely turning off iMessage on both sides. - Telling the phone temporarily to send SMS instead of iMessages does not work.