Question:  What are the pin outs for the 4-wire intercom channels "O" and "X" on the DX-300 (MB300) base station?  Are these intercom input and outputs adjustable from the base station? 
Answer:  The DX-300 (MB300) base station supports two 4-wire intercom feeds, the intercom channels are accessible from the rear panel of the base station via a 10 pin Phoenix style terminal strip.  The channel pin outs are noted below:









The front panel of the DX-300 base provides separate recessed input / output screwdriver adjust of channel "O" and "X" as shown below:






The user accessing either BP300 beltpacks or WH300 headsets can talk to these intercom feeds via the O-channel only button, X-channel only button, or both channels simultaneously by pushing the "ALL" button. 
The DX-300 basestation supports- 4 full duplex BP300s in single channel mode - 3 full duplex BP300s in two channel mode - upto 15 BP300s in push to talk mode. (either single or two channel mode)