Sending SMS on the router - how is it possible?

Question

SMS

Good evening !
Colleague's situation.
- on the Tele2 network, a branded router of the brand OSH-150 - a separate device with one button that distributes the Internet via WiFi
- suddenly outgoing and incoming SMS are seen in the detail (billing), which in total (look like a meaningful correspondence for the provision of money transfer services) lead to write off a small amount in favor of a certain Megafon number
- the SMS functions are not indicated in the documentation for the router, although you can find such data on the network

Tell me:
- which way to look in the sense of reasons?
- "hole" in the router?
- attack vector?
Have there been similar cases?
Tele2 itself in its purchases of client devices exposes the "security" section, but maybe. not everything you need?