I don't think there's a way without a PBX. I'm thinking of setting one up at my house actually at some point. I keep getting this one telemarketter that calls with a fax every couple days at totally random times of the day or night. Sometimes at like 5:00am even. I'm usually in very deep sleep at that point, but if I happen to be awake and hear that, I'm forced to go see in case it's a family emergency or something, then I pick up and it's a bunch of beeps. Pisses me off.