IPtables Netfilter routing project Quick project

I have a FreePBX server and I need to route all VOIP traffic by port. 5060 and 10000 through 20000 through 1 gateway and all other ports through another gateway. All in same subnet same interface. Our ITSP (Spectrum enterprise) will not allow anything other that VOIP traffic through their Voice network. Diagram enclosed. Something like this. [login to view URL] I'm just not an IPtables expert. PLEASE do not bid on this job if you do not know how to do this.