A modem includes modem circuitry and modem software that is executed by a processor. When the modem circuitry detects that the modem software is nonfunctional, it enters an on-hook state to prevent blocking of a coupled telephone line. A nonfunctional state of the modem software is detected when the modem software ceases to interact with the modem circuitry in an expected manner. In a first operation, the nonfunctional state is determined when the modem software does not reset a count down timer in the modem circuitry before the count down timer reaches a termination value. In a second operation, the nonfunctional state is determined when the modem software does not access the modem circuitry before the count down timer reaches the termination value. In a third operation, the nonfunctional state is determined when the modem software ceases writing transmit data to DMA memory.