The Robocall Spam Filter provides methods for telephone subscribers to provide feedback when they receive an illegal robocall. This data is used to generate a blocklist which can used to prevent further such calls. Valid, legal calls can be excluded from the blocklist by a) restricting reporting to only subscribers whose telephone numbers are already in the National Do Not Call Registry and b) maintaining a whitelist of authorized robocallers such as political organizations, etc. and removing those numbers from the blocklist each time before it is published.

The process can be automated so that blocklists are generated and distributed to telephone (PSTN and VOIP) service providers automatically. Optimally, this process would occurr quickly and lists would be updated at least several times a day.

The situation is made more complex, however, by the fact that robocallers often spoof their Caller ID and they may use VOIP services which are either lax in enforcing the law, or are complicit. Another complication is that service providers don't always cooperate by forwarding complete information when calls traverse multiple networks.

To address these issues, we suggest a combination of technical steps which can be taken to improve validation of caller identity by improving the data which can be logged about illegal robocalls in both SS7 and SIP domains. We also discuss operational methods, which can be taken against telephone operators and network entities which do not pass complete or correct data down stream on the network, or which fail make any effort to stop illegal robocalls.

These methods include, in addition to blocking: throttling new or uncooperative operators (PSTN or VOIP providers) or network entities (such as VOIP proxies). Call screening functionality is also discussed, which would require that callers originating from untrusted operators or networks be required to press a key and record a message which is played for the subscriber to allow them to decide whether to take the call.

The Robocall Spam Filter proposal suggests technogical, operational and regulatory solutions, describing a system can be rolled out without major changes to the existing infrastructure and which, if implemented, would stop robocall spam.

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