Could you restate your question? Assuming unity is at the main site (unity itself doesn't support SRST-style fallback to an IOS gateway), callhandlers work as long as the call that gets routed to unity can be classified (according the the call routing rules which take ANI/DNIS/forwarding number into account). This isn't any different than normal operation (unity doesn't know that the remote site has gone offline) except that the remote site would now possibly be routing their calls via the PSTN to Unity.
If I misunderstood your question, please just repost with some more details. Thanks.