From Mexico.purplecow.org
I'm really not sure if this is technically correct or perfect, but it seems to get the job done. Tested with Asterisk and 1760V with VIC2-FXO.
voice-port 3/0 supervisory disconnect dualtone mid-call input gain 10 cptone AU timeouts call-disconnect 0 timeouts wait-release 1 timing hookflash-out 50 connection plar <your sip ext> impedance complex1 busyout action graceful