G.703 interface is either your regular RJ45 interface or a BNC interface.
Normally most 2 Mbps links come as G.703 interfaces unless otherwise requested. If you require to connect such a link to your router, you have 2 options:-
1) Since the link is presented in G.703 RJ45, you need a G.703 module in your router and then you can directly connect the link to your router.
2) If you have a non modular router, the serial interface is V.35, in which case you require a modem(Like RAD or Loopcom) to convert the G.703 to V.35 and then you can connect the Modem output to Serial port of the router.