I'm assuming branches are already networked. If this is a UCD HUNT GROUP (agents do not have to log in) it's as simple as just adding the other extensions. Within the HG go to members, right-click to add, then select IP/DIGITAL OFF NODE DEVICE and pick your phones.
If it's an ACD group you'll need both ACD and REMOTE ACD license on each node, then build the ENTIRE LIST OF AGENT IDs at each site, then pick the IDs to use. ACD AGENT IDs are created in HUNT GROUP RELATED INFORMATION.