Telephone spam costs United States consumers $8.6 billion annually. In 2014, the Federal Trade Commission has received over 22 million complaints of illegal and wanted calls. Telephone spammers today are leveraging recent technical advances in the telephony ecosystem to distribute massive automated spam calls known as robocalls. Given that anti-spam techniques and approaches are effective in the email domain, the question we address is: what are the effective defenses against spam calls? In this paper, we first describe the telephone spam ecosystem, specifically focusing on the differences between email and telephone spam. Then, we survey the existing telephone spam solutions and, by analyzing the failings of the current techniques, derive evaluation criteria that are critical to an acceptable solution. We believe that this work will help guide the development of effective telephone spam defenses, as well as provide a framework to evaluate future defenses.