We discussed on Understanding the Mobile Phone earlier. For a Mobile phone to get connected, the carrier is an important factor.
A carrier is a Mobile operator or service provider, who provides the communication service between the phones.
If a person A calls to a person B, the call reaches the carrier provider , say for example AT&T.
Now AT&T gets a request from the person A Mobile number, checks the destination number and carrier provider. If same carrier provider it transfers the call to the person B phone number and hence a connectivity is established between A and B. If the carrier of B is different, it transfers the call to that different carrier provider and in turn the provider transfers calls to person B. Now A and B will be connected.
This is only a logical way of understanding. The carrier providers follows standard telecommunication technologies for establishing connection between mobile phones.
Hence for maintaining the telecommunication service , and using the service, the carrier provider charges the subscriber or the user with a tariff or plan. The plan varies from region or state. Some plans are also common throughout the country. The carrier provider segregates the plan based mainly on usage and affordability. It has prepaid and post post paid tariffs or plans. A call is billed per minute wise or even seconds wise. There are also free calls, which the carrier provider, charges and waives off a fixed cost, as an offer or promotion. Apart from voice calls, the carrier provider also offers data plans for internet.
So a carrier is needed for connecting the mobile phones and even the wired (landline) phones.
A Sim Card is used as the medium to connect the Mobile Phone with the carrier provider.
The top wireless carriers in US are:
Verizon Wireless: 147.2 million users
AT&T Mobility: 138.8 million users
T-Mobile US: 70.7 million users
Sprint Corporation: 53.7 million users
US Cellular: 5.0 million users
You can refer https://en.wikipedia.org/wiki/List_of_United_States_wireless_communications_service_providers for more information.