I don't think they have to fulfill the call. I can block a number from calling me - and they don't have to let the call go through. They don't have to let folks harass.
It isn't even a technical issue: "mobile companies can in fact block most robocalls but choose not to, in large part because it costs money to do so and at the same time they make money from those calls." (from the article).
In other words, they can still allow me to choose to block those calls, but it seems they only really offer it to non-prepaid customers, charge a fee for the service, and don't exactly advertise that they have the ability.
It isn't even a technical issue: "mobile companies can in fact block most robocalls but choose not to, in large part because it costs money to do so and at the same time they make money from those calls." (from the article).
In other words, they can still allow me to choose to block those calls, but it seems they only really offer it to non-prepaid customers, charge a fee for the service, and don't exactly advertise that they have the ability.