Mobile service providers often disconnect services due to repeated non-payment or late-payment of phone bills. Other reasons include rooting your device or misusing the service through harassment, obscenity, fraud or any kind of illegal activity. Temporary disconnections can sometimes be granted on request and the exact terms vary between providers.
Some mobile service providers, such as Virgin Mobile, reserve the right to suspend or terminate services at any time without notice. Catch-all clauses such as these allow providers to disconnect customers for many reasons - even rooting a phone may result in the provider disconnecting the service. T-Mobile provides examples of permitted and prohibited uses of the service in its terms and conditions. These include any uses that adversely affect the company, their network, their customers, their employees, their business or any other person.