Partyline: Why does my RS-703 Belt Pack only work on Channel "B" when connected to a standard Clear-Com PL system.

The RS-703 (and also the RS-603 and 503) BP requires a TWC-701 multiplexer box to put both ( "A" and "B" from a standard Clear-Com PL system) PL channels on to a single 3pin XLR connector.
If you plug straight in from a PL channel, you will only get that channel on the the 703's BP's Channel "B", as the audio from pin 3 from that single (un-muxed) channel will go to the "B" side and the 703 is expecting to see the "A" channel on pin2, where, in normal cases, there would only be 30v DC power. But the TWC-701 adds the audio from the "A" side of its inputs to pin2 and if it's not there, the 703 will only manage to interact with its "B" channel
See other solutions on TWC-701, RS603 operation