Connecting 2 x G.703 serial interfaces without in-line modems

I have two cisco 2600 routers.

I want to connect together one serial interface on each router using a male to female cable.

Can I run Frame Relay/X.25 over this cable without in-line G.703 modems?

I know it works with in-line G.703 modems as I have used this setup in a standalone config for testing routers that use 2Mbps circuits.

I'm aware that signal timing control is important.

If this is possible what config changes are required on the serial interfaces?

Any help is appreciated.

Th=D8r