SoK: Everyone Hates Robocalls: A Survey of Techniques Against Telephone Spam
Voice over IP telephony reduces communications costs, but it is also subject to spam, i.e. unwanted, unsolicited calls. This paper discusses the problem of spam over IP telephony and reviews published techniques to reject likely nuisance callers. The techniques investigated include content filtering, black lists, white lists, gray lists, call rate monitoring, IP/domain correlation, reputation systems, consent-based communications, address obfuscation, limited-use addresses, computational puzzles, Turing tests, payments at risk, legislation, centralized SIP providers, circles of trust and authenticated identity. The advantages and disadvantages of each are analyzed. It is concluded that no single technique is sufficient and that a framework of multiple techniques is required. One proposed framework is analyzed and it is found that it has limited support for voicemail spam and contact request spam.