I have a sip account which only allows one device to be registered. When i'm at home I want incoming calls to be able to ring on multiple devices. All of these devices are connected to the local network. I'm guessing the way to do this is using a local server/proxy that would allow multiple registrations which then forwards traffic to/from my sip provider. What a simple way to do this on either OS X, Ubuntu or using some low cost SIP router hardware?