Connecting multiple freepbx boxes


I want to connect 3 freepbx boxes. One box that has a trunk to the outside world with 8 channels. Two other boxes should connect to the gateway each with 4 channels. I can create trunks between the gateway and Office1 but what i can’t seem to manage is to call to the outside with Office1. My softphone connected to an extension on the gateway can call to the world. I have no specific dialing rules yet. All i want is for extensions on the Office1 machine to call the world. I have been trying this with trunks but maybe i should create extensions for this on the Gateway? If anybody could point me in the right direction i would be very grateful

Whats the Gateway ?
How are you connecting to it - SIP, H323, IAX ?
What you are doing is straight forward but we need more information.