A modem which includes connections for both land lines and a cellular phone. The modem contains high and low level routines that allow it to perform standard AT commands rationally when connected to a cellular phone, and further perform additional AT commands that access cellular specific features. An applications software program in a computer connected to the modem can provide a number of options for determining whether to use the land line or the cellular phone when both are connected. First, it can default to the land line and only use the cellular phone if the land line is not available. Alternatively, it can first use the cellular phone and only use the land line if the cellular phone signal strength is not sufficient. Further, cellular file transfer operations can be aborted if the remaining battery life in the cellular phone is insufficient to reliably complete the transfer.