I suspect people on extremely limited plans know to screen their calls or manage in some way.

If not that is unfortunate but hardly justification to upend cellular calling because a few people paid $0.10 / minute extra answering calls from spammers.

There are advantages and disadvantages to those types of plans, and that is one of the disadvantages.

As far as I know all of Verizon's prepaid plans offer unlimited talk and text.

Sure I can. Many pre-paid plans also offer unlimited minutes, and even in the case of those that don't, you can absolutely just choose not to answer the phone if you don't recognize the number, or just flat-out don't feel like answering.

Look, I get that the US is different in that it charges for incoming calls, and that's weird to you. That's fine. I think it's weird that Spanish people voluntarily let themselves be chased by enraged bulls, but hey, I let people choose the risks they want to take[1]. Would I prefer free things over not-free things? Sure. But that's the model that US carriers chose, and, absent government regulation to the contrary, it's absolutely their right to choose that model. And I honestly just don't care enough about it to lobby the gov't to change those regulations, and it seems that a majority of Americans feel the same way, so... so what?

[1] Modulo unethical treatment of the bulls themselves, but that's another issue.

If I'm X telecom company and I want to inject a billion roboscam phone calls into AT&T's mobile network, I'm going to pay them for that privilege.

These phone calls are no longer expensive, generally speaking, which is one of the reasons it is now cost effective.

The access tandem is where long distance calls are switched into the local phone network for a given LATA. Local phone companies and long distance providers all buy into the same access tandem in order to connect into the geographically fixed market. When calls are switched across an access tandem in the US, it's the initiator of the call that is billed per minute. The receiver at the other end of the tandem switch may pay a fee to the switch operator to receive the RECORDS of which other company initiated a call in to them. That way, they know who to go after and bill for each incoming minute! Tandem operators typically don't provide this info during the call signalling, even though they could.

As you can imagine, per-minute call billing is not always settled between tandem players. They can block each other based on company ID alone, and often this requires cooperation from the tandem operator as well. Some of the per-minute rates are incredibly high. The flat-rate long distance phone providers are obscuring the fact that some areas are extremely high-cost to terminate. This system has sprung into high per-minute cost billing in high-cost areas (rural LATAs) so that people operate free dial-up (in the 90s anyways), free phone conference lines, and even "free international calls" all taking advantage of the incredibly high per-minute rate by billing the maximum rate permitted in their LATA (typically not higher than the highest per-minute tariff posted by the incumbent local monopoly).

This highly regulated old-school system seems so antiquated in the face of the Internet, but it survives because the tandem is the one place where all providers can interconnect, and this turns out to be the hardest part of the puzzle to work around. In the age of the internet, we could come up with something better :)

This is a great point - and was the primary idea behind charging for sending email by the message to combat spam. Someone years ago proposed that if we just charged 5 cents per message sent, most people would pay less than $5 a month while spammers would have to pay millions of dollars of the course of a year, making the business unprofitable.

The caller also may have to pay, depending on their arrangement with their carrier. Having the receive pay or not pay has no bearing on that.