There are many different ways to make branch office connectivity with multiple trixboxes, here are some good guides:
http://freepbx.org/news/2006-10-25/connecting-two-freepbx-machines-together
http://freepbx.org/support/documentation/administration-guide/connecting-2-or-more-boxes
http://freepbx.org/2006/10/26/connecting-two-freepbx-machines-together
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