I guess you can do it this way..
Create an ICT between 2 of the CUCM clusters. For Remote office 1 users which are registered to Remote office 2 CUCM, create a unique dialing pattern for dialing out locally from Remote Office 1 & route this pattern back through the ICT to a PSTN router at Remote office 1. Not the best approach though but that's what I can think of at the moment.
An ideal approach would be to have these 2 remote sites part of single CUCM cluster & deploy the UCCX HAoWAN setup. That way, both side users are part of UCCX & have their own local dialing facility, the simpler way.
Hope it helps ?
Please rate helpful posts.
GP.