NSA et.al. work in secret, outside the law. Formally they're covered by the law, but the problem is that this includes many secret laws giving them lots of leeway, and if the law gets in the way they'll ignore it anyway.
The FCC and phone companies however work more in the open, and are bound by the law. One such laws says that the phone company must do their best to make all phone calls come through, no matter the content. This is typical part of being a common carrier (like the postal service): they can not discriminate against content, they have to put through the message, and also are not liable for any content of the message.
While technically easy, it's legally not so. The phone company must put through those calls, even if they know this are robocalls and the customer doesn't like robocalls. The customer however is free to install blockers on their phone, or to have their calls rerouted through a third party which helps them filtering the calls. This is exactly how it's done now.
Finally, you should really equate this with Internet filters. They're hated here - yet phone filters are asked for. Phone filters can also be used to block political rivals and let your own calls go through, for example. That will give your party an edge at the next elections.