What is the +0 in front of the received SMS number?

In the past, due to the lack of certain norms between operators and branches in various provinces and cities, sometimes there was a +0 (at that time, it was generally a short message between non-local numbers) and sometimes there was no (message between local numbers).

Now it has been standardized, so no matter what number it is, as long as it is sent between mobile phone numbers, the displayed number will be preceded by a +0. Of course, you can receive it face to face without sending +0 when sending it.