How to send a notification to a subscriber?

Question

iOS

Good day.
I will try to describe the situation in as much detail as possible.
Today, a character sent me an alert on my cell phone. The screen just lit up, and in the middle of it was a message, and at the bottom there was a close button. As when requesting a balance from the operator. This message has not been preserved anywhere, there is no information about it in detail. And the internet was turned off on the mobile.
Phone iphone 4s, 8.1.2.
What kind of service is this and how is it implemented?

Answer 1

Flashsms is called, he figured it out. Thanks to all.

Answer 2

if it is mts, then from the official website, see the "Send SMS" section

Answer 3

well, there are a lot of options, where are the screenshots, what sounds, is it on a locked screen, etc.