Standby time is the number of hours that a freshly charged battery will keep a mobile phone operating for when the phone is not in active use.

The maximum length of time a wireless phone or communicator is fully charged, turned on and ready to send and receive calls or data transmissions. Standby time is reduced by the amount of time the pho [..]

The amount of time you can leave your fully charged cellular portable or transportable phone turned on before the phone will completely discharge the batteries.

The maximum length of time that a device is fully charged, turned on and available to send and receive calls or data transmissions. Transmitting calls and data reduces this time because transmission requires more power.

This refers to the number of hours that a mobile phone battery will last without receiving or making any calls. If you use the phone to make calls or send messages then the standby time will be diminished.