Modem dialback and ct program

[cctal!andrew at relay.eu.net]

Does anyone have a shell script or C program for implementing "dial-back" modem 
security? 
Also, the machine on which I wish to implement this does not have the "ct" 
program. Does anyone know of a PD version of this or a way round the problem 
(i.e. to be able to call a user and present them with a login)
Thanks 
Andrew Hardie
London, England
ukc!cctal!andrew
P.S. System is Altos 2086 running Xenix.