Smartphone-based electronic contact tracing is currently considered an essential tool towards easing lockdowns, curfews, and shelter-in-place orders issued by most governments around the world in response to the 2020 novel coronavirus (SARS-CoV-2) crisis. While the focus on developing smartphone-based contact tracing applications or apps has been on privacy concerns stemming from the use of such apps, an important question that has not received sufficient attention is: How reliable will such smartphone-based electronic contact tracing be? This is a technical question related to how two smartphones reliably register their mutual proximity. Here, we examine in detail the technical prerequisites required for effective smartphone-based contact tracing. The underlying mechanism that any contact tracing app relies on is called Neighbor Discovery (ND), which involves smartphones transmitting and scanning for Bluetooth signals to record their mutual presence whenever they are in close proximity. The hardware support and the software protocols used for ND in smartphones, however, were not designed for reliable contact tracing. In this paper, we quantitatively evaluate how reliably can smartphones do contact tracing. Our results point towards the design of a wearable solution for contact tracing that can overcome the shortcomings of a smartphone-based solution to provide more reliable and accurate contact tracing. To the best of our knowledge, this is the first study that quantifies, both, the suitability and also the drawbacks of smartphone-based contact tracing. Further, our results can be used to parameterize a ND protocol to maximize the reliability of any contact tracing app that uses it.