You can. It's called a credit card.

It would work differently with physical goods, rather than a service that is practically just there, as each unit of goods has a cost that must be recovered. Now, one could give reasonable unlimited access to a loss leader good, with the expectation one would purchase other goods on a normal per item basis.

With a service such as mobile phones, the ongoing cost is small, they just need pay for electicity, outside ,network connections and the amortized cost of the hardware.

come on, you know why it can't work. all you can eat restaurants are ok 'cos you can't eat that much, but a fixed price supermarket...no chance.