Alan, the way that I understand it is does not work from Cisco gateway to clarent gatekeeper. The way that works is registering your cisco gateway to a cisco gatekeeper. Then you make the cisco gatekeeper talk to the clarent gatekeeper.
This is all theory, since I never did it myself. I will be trying to do the same in few weeks, for that reason I am very interested in what you find.
Will you mind sending my a e-mail??? firstname.lastname@example.org.