In many other national numbering plans outside the NANP, mobile services are assigned separate prefixes. Cell phone numbers in the NANP are allocated within each area code from special central office prefixes and calls to them are billed at the same rate as any other call. Consequently, the "caller pays" pricing model adopted in other countries, in which calls to cellular phones are charged at a higher nationwide rate, but incoming mobile calls are not charged to the mobile user, could not be implemented. Instead, North American cellular telephone subscribers are also generally charged for receiving calls ("subscriber pays"). This has discouraged mobile users from publishing their telephone number. However, price competition among carriers has reduced the average price per call minute for contract customers for both inbound and outbound calls, which compare favorably to those in caller-pays countries. Most users select bundle pricing plans that include an allotment of minutes expected to be used in the billing period. Of the four major national carriers in the United States, all four (AT&T, T-Mobile/Sprint, Verizon) offer free calling between mobile phones on the carrier's network, and Sprint also offers its customers free calling to mobile phones on other networks.