Yes, all SFB server have to reach each SFB server for communication and also all SFB clients for peer-to-peer calls and conferences hosted on the other standard server.
Only way will be to have one SBS server in each branch office for HA without conferences services, but the SBS server have also to communicate together.
If the clients can not reach each other, they will try to use the internal NIC of your SFB edge server.
regards Holger Technical Specialist UC