Depending on your plan, you may be either paying for minutes or they're using up your allotment, which in some cases means you could end up paying a premium rat

Depending on your plan, you may be either paying for minutes or they're using up your allotment, which in some cases means you could end up paying a premium rate for going over. If you're on an "unlimited" plan, this bit is irrelevant but at the same time, the marginal cost of delivering calls is essentially zero once the network capacity exists. If you're roaming on another carrier's network, I'm not sure how the economics work there, but I suspect the other carrier gets paid regardless.