I had the same issue on my phone. It turns out this is a side-effect of the NOMOROBO service that Comcast has connected via a 3rd party partner. If you have this enabled (you can see this from your Xfinity account under phone>Settings>Call Forwarding), I was told by tier 2 support that the phone will ring once - allowing the NOMOROBO to look up the caller - then immediately drop the call. NOTE: No caller ID is even visible during the one ring.
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