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Modern mobile phones are equipped with infra-red photoelectric proximity sensor, most commonly applied to turn off the touch screen during a phone call to prevent accidental touches when users face/ear is detected in proximity to the screen. In this project we propose to achieve the sensing functionality without using a special sensor. Specifically, we use an already existing speaker and microphones for proximity sensing, without interfering with their originally intended operation. We build our method on the observation that the transfer function from the mobile phone speaker to the microphones varies as a function of objects located in the vicinity of the mobile phone. We have prototyped our solution on 2 mobile phones - Samsung Galaxy A70 and Xiaomi Mi 9, and achieved 99% accuracy of detection. We demonstrate that our method is robust to various environments, geometries, fabrics, and performs well in the presence of noise. We also evaluate the performance of various implementations and discuss their trade-offs.