The audio trunking cable shown below allows for audio trunking between two Eclipse (or Matrix +3 frames) or it can be used to link two EF-701M interfaces together directly.  Only the audio pairs are used when trunking two frames together, the RS-422 pairs are not needed or they can be looped back to themselves (pin 1 to pin 7, pin 2 to pin 8) in order to have the frame recognize that the frame is being used and activate the port light. 
A standard CAT-5 patch cable can be rewired at one end to make an audio crossover cable