This was likely someone running a test / dev network using OpenBTS, which is open-source software for GSM base stations. The exact phrasing of that message is the default registration message for the software: https://github.com/ttsou/openbts-p2.8/blob/master/apps/OpenB...
2G networks have basically one way auth. The phone auths to the network but the network doesn't auth to the phone. This is an oversimplification but basically how it works.
This is not entirely true. LTE is authenticated. If you force your device to not allow any base station using an older protocol (to prevent downgrade attacks), you should be fairly safe. The problem is that most phones will accept a base station that says "No, sorry, can't do LTE, how about this unauthenticated 2G AP instead?" If you are ok with your phone not having network in rural areas, setting it to LTE-only is a reasonable mitigation (assuming your device supports this).
Some of the other comments are that a 2G tower tells a phone whether it's allowed on or not, but that the phone doesn't have any verification in the other direction. That's just the design of 2G cell networks. It's not an optional setting.
Other comments say that LTE networks authenticate in both directions, so you're safe if you can set your phone to LTE-only.
