The 400EM module supports Type I and Type 5 format, which will also partially determine your wiring scheme. Type I uses a Transmit pair, a Receive pair, Ear, Mouth and a Signal Ground (ground is absolutely, positively critical). Type 5 simply uses two pair and is more popular in the U.K. and such.
The signaling format is set by the DIP switches in the front of the module. See pages 2-35 through 2-38 (Adobe Acrobat pages 100-103) in
https://support.avaya.com/edoc/docs/legend/leg61in1.pdf to determine what the positions mean.
Let's start there. I'm looking for a wiring diagram. If one can't be found, I'll show you how to use a multimeter and a lineman's test set to determine the leads.
