Same in mine, spammers buy blocks of numbers (for example all numbers from XXXXX100 to XXXXX999) and use them to avoid being blocked by people who block individual phone numbers (with the default phone app). Fortunately I found Yet Another Call Blocker on F-Droid and I can block by pattern with it (XXXXX*) but most less technical people won't be doing that.

You don't even need that, at least in America. Phone spoofing is terrifyingly easy.

Do you not have to pay a network provider in order to use it to make calls and send messages? How does this work?