We are having the same issue, and the answer I got from TAC (at least for right now) is that it is not supported on the FTD yet. I'm pretty disappointed because we need to keep our old ASA 5520 around just for the VPN phones now until they get the code working on the FTD. Here's the response I got from TAC on it. The bug he mentions below is CSCvf88533, but it appears to be internal only so we can't view it.
In order to create the Phone-VPN-Trust certificate in CallManager the identity cert from the FTD has to be exported into CallManager. There is a feature enhancement request to allow certificate exports in the link below but I don’t see much visibility on this issue so this may be pushed towards end of the year. I only have visibility out a few months and do not see this as being released any time within that time frame. Unfortunately FTD is still a new product and we are racing to get features implemented. As of now on FTD we have a bit less than 1/10 of the features available from ASA so this is an ongoing struggle. I have added your case to the enhancement request to gain more visibility. In the mean time you can track the enhancement request on Cisco and be notified via email with this enhancement request has been resolved.
