Comment by dredmorbius

There are a number of options. My view is that carrier-based filtering (rather than on-device filters) are where effort must be focused. Much as we learned with email: if you're routing traffic for many people, mass-contact attempts and patterns become quickly visible. Individuals see only a minuscule fraction of traffic, networks see overall patterns.

The other element is that carriers can act at the network level, noting how much abusive traffic arrives from given peers, and taking direct action against those peers. That could involve rejecting traffic outright, subjecting it to stronger challenges, and/or diverting it to investigative / law-enforcement bodies (I'd suggest both national and state entities) for both tracking and enforcement. Power-law relations mean that at any given time, a small number of networks will account for the overwhelming majority of spam, though which networks will likely change over time.

The key problem with this is getting the carriers to act, which ... will probably involve a few carrots and sticks. I'll address those in another comment, except to mention bonding: <https://oag.ca.gov/consumers/general/telreg>.[1]

Individual action will not solve this problem, but there are steps you can take.

Most major US carriers now offer some form of robocall blocking. "Scam Shield" from T-Mobile, "ActiveArmor" from AT&T, "Call Filter" from Verizon.

MVNOs (mobile virtual network operators) may or may not offer scam / robocall blocking themselves (though IMO they should, and should be required to). Some will identify spam calls, but those are still passed through to your handset.

Beyond this, there are on-device apps which can be used, some are carrier-based (e.g., "Call Filter Plus", from Verizon, similar tools exist for Verizon and AT&T), some are third-party. These of necessity share your voice/text activity with third parties, which is its own concern and consideration.

Full Android, iOS, and several full-featured Android alternatives (GrapheneOS, /e/OS, LineageOS, etc.) offer unknown caller rejection. Numbers not in your contact list are directed to voicemail. At present, few spam calls will leave voicemail, though some do, and as AI expands in capabilities, applications, and adoption this will all but certainly increase. I'd strongly encourage use of this.

Feature phones / dumbphones ... have far less capability. Most cannot even reject unknown numbers, which ... seems a ripe target for legislation and/or regulation. Phone frameworks such as AOSP / KaiOS seem to afford little capability for even creating a call-blocking app. This and other dumb devices (e.g., traditional landlines) are a strong argument for carrier/network level mitigations.

The company everyone loves to hate, Comcast/Xfinity, actually has one of the most sophisticated voice/text spam blocking systems, and one I'd like to see mandated to all carriers: <https://www.xfinity.com/support/articles/spam-blocker-overvi...>

It's risk based.

It classifies calls into three categories: high, medium, and low risk.

It adjudicates calls based on risk.

High-risk calls are terminated entirely.

Medium-risk calls are directed to voicemail.

Low-risk calls are subjected to an audio CAPTCHA (enter a two digit value to ring through), otherwise are directed to voicemail.

(It's not clear whether or not a whitelisted number will escape any treatment, perhaps subject to conditions such as originating from the appropriate/approved network for that call.)

I haven't used that system, but in advising people still moving off landlines, or looking at VOIP solutions, it's making Comcast an attractive option.

(I don't know what other VOIP providers, say, Twillo or Asterisk, offer, but suspect at least some have similar if not more-capable systems.)

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Notes:

1. California requires a $100,000 bond by all telemarketers in the state. The state has a small fraction of the incidence of robocalls of the worst US states. Several others have some bond. My view is that bonding should apply at the carrier level and be surrenderable to both contacted individuals and downstream peering networks, to provide both a strong financial penalty to abusers, and an incentive to downstream networks to pursue abusive calls.