You can make sure how you establish the call.
1.) Did your RP register to GK server . Same as the Remote Site
2.) You call by IP address ? Is the IP address of the Remote Site is able to Reach ?
3.)Can the media Pad control others function of the RP beside make call ?
4,) You can share what is the error message appear in the Media Pad when you fail to make a call.
5.)If possible, you can share the basic network topo of your video conference base on current case.
Thanks