How to use a mask to check for operator codes in the phone field?

Question

JavaScript

There is a form with a field for entering a phone number, which provides input by mask. For this purpose, I use the imask.js

library. In connection with this, 2 questions that interest me:

How to check for operator codes from 700 to 709, 747, 750-751, 760-764, 771, 775-778 inside brackets ()
How to format the resulting phoneMask.unmaskedValue as [phone] not outside the mask options (as I did now) but inside at the level of the mask itself?
let valueWithPref = "+7"+phoneMask.unmaskedValue;