idle rainy day pondering - cheapest way to have a phone number + a little data for a backup iPhone?

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https://www.xfinity.com/mobile/

my bill was $2.74 last month (that was entirely taxes and fees, comcast actual bill was $0.00). 100 MB of data, unlimited talk and text, Verizon network, and if I exceed 100 MB, $12/GB.