The Wake on Ring feature is designed to wake the computer from sleep when a phone call is received by the internal modem. After the computer wakes up via this feature, it will not go to sleep again automatically. When a modem is set to receive a fax, the computer will not sleep because the serial port is busy monitoring for an incoming phone call.
All of the PowerBook 100 series computers (except the 150 and 190) support the Wake on Ring feature. This includes the PowerBook 100, 140, 145, 145B, 160, 165, 165c, 170, 180, and 180c.
PowerBook 500 series computers equipped with a Global Village PowerPort Mercury modem also support the Wake on Ring feature. (The Wake on Ring feature is activated through the PowerPort control panel.)
All other PowerBook computer models (including the 150 and 190) do not have this same "Wake on Ring" feature. Instead they have a "Wake-up At" feature which wakes the computer from sleep at a specified time rather than when the phone rings.