SIP ´steal router´ and internet bridge

Please, check out the picture.
viewer.png
I want to ¨catch¨sip packets and forwarding them to lan, while the internet packets (no sip) forwarding them to company router without any modifications.

Any idea?

The idea of functionality is like a Medatrix eSBC from Panasonic
5.3_eSBC Mediatrix.ppt (1.05 MB)