I never did this so I must be missing something, I tried both below but the bridge then fails. Can anyone shed some light on what I am doing wrong. My ITSP now supports in-reply-to so I can pass the caller ID to a forwarded call from FS.

In-Reply-To should contain a Call-ID not a caller ID. They are very different.

The following would make a bit more sense, but still not a lot:

<action application="set" data="sip_rh_In-Reply-To=${sip_call_id}"/>

Using In-Reply-To in a response doesn't seem right to me. I would expect it to appear in an INVITE. For for example, you get a missed call and you call the person back, then the INVITE for the callback would have a new Call-ID but the original Call-ID in the In-Reply-To header. That said, I have no idea what your ITSPs intended use for the header is.

Richard