A) How is it related? B) I cannot just in my phone select caller ID I want, "phones in Poland allow to call you 'from'" is not true. It's just spoofing as in anywhere else and requires non-trivial technical knowledge.

This works in many countries, since the signalling protocols historically assumed a trusted small set of participants, not unlike email – with similar consequences once those assumptions became less and less true.

I constantly get scam calls from numbers that are very similar to my own in Canada. I assume this is an attempt to look like a normal trustworthy number.

it’s common for cheap esim providers to route data etc through cheaper data exits, i imagine this is partly why.

(I recently purchased an esim and was surprised to see it exiting poland instead of the country the mobile provider (Bell) resides in)

I've worked a bit on the app which calls major telco provider directly. It was a basic web service call, and sender could be entered as anything. This is basic property of cellular networks, no more safety than say standard email.