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Telephony systems are imperative for information exchange offering low cost services and direct reach to million of customers. They have not only benefited users but have also provided a convenient medium for spammers. Voice spam is often encountered on telephony, such as in the form of an automated telemarketing call asking to call a number to win million of dollars. A large percentage of voice spam is generated through automated systems which introduces the classical challenge of distinguishing machines from humans on the telephony. CAPTCHA is a conventional solution used for distinguishing humans and machines, and audio-based CAPTCHAs have been proposed as a solution to curb voice spam. In this paper, we conduct a field study with 90 participants in order to answer two primary research questions: how much inconvenience does CAPTCHA cause to users, and how different features of the CAPTCHA, e.g., duration and size influence usability of CAPTCHA on telephony. Our results suggest that currently proposed CAPTCHAs are far from usable. We provide certain guidelines that may help improve existing CAPTCHAs for use in telephony systems.