New mobile codes are capacity, not new operators. What the numbers tell you and what they do not.
How codes are issued
When an operator exhausts its ranges, it receives a new code from the regulator's reserve. That is how 958, 977 and 999 appeared — they extend existing operators rather than introducing new ones.
What 9xx means
All Russian mobile numbers start with 9; the next three digits are the code, split into ranges owned by operators. A code can be shared between several companies, especially an old one.
Finding the operator
The code page lists the operators and the number of ranges each holds. If one holds nearly all of them, the answer is unambiguous. Otherwise check the full number.
Why a code is not exact
A code only narrows the list. The exact operator comes from the range that contains the digits after the code.
Codes never change
A code stays with the number for its lifetime: moving house, changing the plan or porting to another operator does not change it. So the code indicates where the number was issued, not where its owner lives now.
See also: 9xx codes of Russia, identifying the operator.