asterisk users - Jul 2007 - no ringback from SIP server when originating call

I have an application that uses the Asterisk Management Interface to bridge
two calls using the Originate command with Dial as the action.

Using one SIP server, there is no ringback on the second leg of the call.
The first person is called, answers, and hears silence until the second
person picks up, even though the second person's phone is ringing.

When the call goes to another SIP gateway, ringback works fine.