A prefix narrows it down to a group of operators; only the full number gives an exact answer.
1. Look at the code
Russian mobile numbers start with 9. The three digits after +7 are the code: 900, 916, 977 and so on. Each code is split into ranges that belong to specific operators. A code page lists every operator that holds ranges inside it, with the number of ranges as a hint of who dominates.
2. Check the full number
The code alone is not enough: a single code can be shared by several companies. A full-number lookup finds the range that contains the digits after the code and returns the operator that owns it. This is the only method that gives an exact answer.
3. Use the official allocation register
Number ranges are assigned by the regulator and published as an open register. Operators may sub-allocate ranges to each other, so the register is authoritative for the original assignment, while the live operator can differ after a port.
What a lookup cannot tell you
The name of the subscriber. No legal service has access to that data — only the operator does, and only to law enforcement. Apps that promise a name are showing crowdsourced labels, not the registry.
About number portability
Since MNP, a number can move to another operator while keeping its code. So the code tells you the original allocation, and a portability check tells you the current network.
See also: 9xx mobile codes, how caller ID databases work.