I think you will find that ARP is not sufficiently reliable. There have been many threads here on how to detect presence and no real conclusion on the best way to do it.
I do it by determining whether the phone is connected to the WiFi, by using fixed IP addresses and ping. Even that is not perfect as a phone that has not been used for a while may go to sleep and fail to connect for some time after arrival. The technique has become less perfect over time as phones have become more aggressive at saving battery usage.
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