Verizon and T-Mobile are two major mobile network operators in the United States with millions of subscribers. They provide mobile services through SIM cards, which customers insert into their devices to access the network. To contact the operator, users can dial "0" for directory assistance. These operators, also known as phone carriers, offer wireless voice and data services to subscribers. In some cases, users may need to contact operators to request a telephone number. However, in 2021, AT&T ended operator services for wireless callers. Customers can still access operators and directory assistance through home phone landlines. Other mobile network operators in the US include Tychron, Union Telephone, United States Cellular Corp, and United Wireless. When switching to a new service provider in the same area, users can keep their existing phone number.