An off-hook state of a telephone associated with a computer is used in order to disable the power management unit of the computer to prevent premature power shutdown while the telephone is being used. A power-managed computer system includes a bus system, and a central processing unit coupled to the bus system. The central processing unit has a normal power mode and a power saving mode. A telephony interface coupled to the bus system has a port for coupling to a telephone system network. A power management unit is also coupled to the bus system and is responsive to bus system activity and to indicia of telephony interface activity. The power management unit causes the central processing unit to be in a power saving mode when both bus system activity and telephony interface activity are less than a predetermined level of activity. Additionally, the power management unit maintains the central processing unit in a power mode greater than the power saving mode when either the bus system activity or the telephony interface activity is greater than the predetermined level of activity. An off-hook signal is directly sampled from the modem and provided to activity detection logic within the power management unit. Alternatively, modem interface logic interprets any number of signals provided by the modem to deliver an off-hook signal to the power management unit. Alternatively, telephone interface software includes an off-hook identifier that records the off-hook state of the telephone and an enablement/disablement register in the power management unit is either set or reset. System or user activity is also emulated in order to indicate to the power management unit that activity is occurring within the computer.