I had a cordless phone
which had a broken ringer
due to my dropping it on the floor one too many times. Since our wired phone
was totally broken at the time, we had no ringer for a few hours until someone decided to go and buy a new phone later in the day. Anyways, I found that the old IBM MWave soundcard
in our computer would emit a tiny pop from the speakers when the phone line was ringing. By sitting in front of the computer with the phone
and waiting for the nearly silent pop I managed to increase my call answering percentage from 0% all the way to a whopping 25%, not bad.
If you're actually in this situation, a computer with a modem can really help, connect using HyperTerminal or something direct to your modem's COM port and wait.
Eventually you'll see it.
Hit talk and there you go.