Ok, so I think I figured it out. Adorn 3430 will be sitting in the same network as my pbx. 3430 will have it's own private address lets say 10.0.0.6 and my pbx 10.0.0.5. All my devices on the network will be connecting to the pbx and the pbx will use this internal private IP 10.0.0.6 to connect to voip provider instead of the usual public IP. So all voip will go out on the 4MB line and my data still uses the 30MB line.
Sorry for the noise here, looks like I just needed to type it all out
Draw it all up but I can't insert an image here.