I can help you with this, or done this or similar several times.

I'm assuming the os7100 does sip peering or registration to the SBC (A download from the system well tell me this).

Solution A;

Get the sbc lan ip address changed to the data network, then patch this into the data switch.
Change the ip address and sip settings as required.
Set private with public and put your internet static ip in, do the required port fowards and off you go.

Solution B;

Get another Ethernet router, set it's LAN ip address as the gateway on the os7100, then connect the os7100 and sbc to the lan side of the router. Set a static ip on the wan side and connect to your lan.
Set private with public and put your internet static ip in,
do the required port fowards in both routers and off you go.