[g_p]

In the US, without an inclusive calls plan (i.e. what you call a contract in the UK), both caller and recipient pay - it appears that you pay for "air time" (time in an inbound or outbound call). Most people will be on a contract with unlimited "air time", which effectively gives unlimited inbound and outbound calls without charge.

Operator billing models are seldom rooted in reality, and often are quite counter productive - 4G originally was a "pay extra" feature, despite the fact it is in an operator's interest to get users onto 4G due to the increased spectral efficiency, and better performance with weaker signals (so less complaints). They designated VoLTE (4G HD calling) as a premium feature for contract users, but it actually reduces the load on the legacy voice network. And don't start me with WiFi Calling, which is also often a contract premium feature, but actually trunks the call over a connection the user already pays for (and avoids using the cellular network!) - common sense plays little part in how operator billing works.

In the UK look up the "termination rate" if you're interested - this was the mechanism through which the receiving side of the call made money and was able to offer receiving of calls for free.