Just a message popup, a window with dark background and some text ad on it.

I did not buy this phone from a carrier, just added the SIM card later.

Really surprised to learn this doesn't happen to others. Always assumed that the SIM card had some special privilege given by Android.

Sounds like your carrier is abusing STK to display ads.

See https://www.browserstack.com/guide/stop-popup-messages-in-an...

Caveat: if they're doing that, then they're almost certainly data mining your data streams (e.g. dns lookups etc.)

I wouldn't feel secure on such a carrier unless I also VPN'd traffic to a reputable provider (Nord, Express, or Proton) and forced DNS over TLS to known servers.

SIM cards can come with apps preloaded. There was a carrier in Mexico that would load a SIM app for Dominos Pizza and you could order a pizza from your phone if you were on that carrier. I learned this because of some carrier certification feedback I had to disposition at one job.