How to disable the input of eight in the phone?

Question

JavaScript

There is a text field with a mask like "+7 (___) ___-__-__".
When you start typing, characters begin to appear in brackets, that is, starting with the operator code.

Some users do not look at what they enter, and enter the phone number there starting from the eight, as a result, they cannot be reached later.

How can I use JQuery to catch the first entered character and throw out an alert if it is a seven, eight or the "+" symbol?

I'm a total noob at Jquery, please help!

UPD: All Russian mobile operator codes start with nine.
The question concerns specifically mobile operators, landline phone numbers should not fall into the database.

Answer 1

And if the client, for example, is from Rostov or St. Petersburg?

Answer 2

When you enter the phone, do you go to the next field? If not, then shift the digits of the number to the left