The letter from the National Association of Attorneys General raises issues regarding the practical and technological challenges that may arise regarding the implementation of certain call-blocking technologies by common carriers to address the problem of robocalls. Many comments – including those from individual consumers – acknowledged USTelecom’s concerns regarding the technological limitations and potential risks to deploying services and tools heavily reliant on black list or white list technologies. USTelecom and others discussed at great length the potential risks associated with broad implementation of black-lists, particularly as they relate to spoofing. For example, many commenters discussed concerns whereby innocent consumers could have their phone numbers spoofed by robocallers, resulting in their legitimate phone number ending up on a black list. The record clearly demonstrates that such concerns are not merely hypothetical. Several commenters also expressed concerns regarding the conflation of “illegal” calls with “unwanted” calls. But as some commenters noted, it would be impossible to even make such a distinction since calls from certain organizations may be reported as unwelcome by some subscribers, while other subscribers may wish to receive them. There is an absence of any reassurance that the systems proposed would differentiate between legal and illegal. This is the primary reason carriers are cautious about deploying such technologies given their inherent risks, their inability to distinguish between “illegal” calls, “unwanted” calls and “legal” calls, and the lack of any Commission precedent directly addressing them, particularly with respect to the potential harms to calling or called parties. The concern about overly broad blocking technologies interfering with legal calls was raised by various commenters. The record confirms that false positives do indeed occur, with one provider of blocking services acknowledging a “small number of false positives,” and another stating that “[f]alse positives and false negatives will undoubtedly occur.” But no party actually submits evidence supporting assertions that the various proposed blacklist based blocking solutions are “reliable.” USTelecom agrees that it is difficult if not impossible to accurately state what the actual number of false negatives or false positives will be for particular services or devices, but that does not justify the failure of proponents of specific blacklist-based solutions to share data about the potential for their services to cause negative externalities such as improper blacklisting of legitimate customers.