Fraudsters are increasingly using SIM cards registered to straw persons. The scheme is simple: a person is offered a reward for a number, they register it in their name and transfer it to others. Then such a SIM card can be used for calls, phishing, and other schemes.
The number is not needed by criminals for its own sake. It can be linked to banks, "Gosuslugi", messengers, and other services, which means it helps to act on behalf of another person and quickly change contacts after blockages.
One of the ways to bypass this is 'dropperism', which involves registering SIM cards to straw persons. For a reward, a person registers a number in their name and transfers it to others, after which such a SIM card can be used for calls, phishing, and other schemes.
A separate problem remains with already activated numbers with other people's or unreliable data, as well as corporate SIM cards that companies do not monitor well. Restrictions on the transfer of numbers and self-imposed bans on their registration complicate such schemes, but do not completely close the market.
You can check the numbers registered in your name on "Gosuslugi" in the "SIM cards" section. It is better to block an unknown number, but first, you should make sure that no important account is linked to it – otherwise, you may lose access to services.