In the normal scheme of things, on a single channel RS-702 beltpack has, as its pin-out pin 1 = common, pin 2 = 30volts power, and pin 3 = intercom audio. The RS-603 two channel beltpacks require a TWC-701 interface. It is a  2-channel/3-pin XLR  adapter module; combines two intercom channels onto one single 3-pin cable for stations with the TW option and is necessary for the beltpack to operate in two channel mode, as it is the only way to combine the two channels on one 3pin XLR. It places the audio for the "A" channel on the powered pin 2, so the extra DC voltage normally applied to pin 3 is not effective on pin 2 which now also carries PL audio for channel "A". so...the RS-703 will have only one call signal from the mainstation side of the interface to the beltpacks, though among themselves,  the beltpacks will have two call signals. Because of the nature of the multiplexing involved, the beltpack will only have full system functionality of call signal on channel “B” to all the beltpacks. 
When using the RS-603 BPs,iIn order to have the beltpacks have channel “A” call among themselves, you will  need to use the V-PAK software( V-PackVersion 1.1.3) available from our website  http://www.clearcom.com/support/partyline-software  to program the RS-603 beltpacks to allow AC signaling on channel “A”. You’ll also need to make a cable from a DB=9 female to 3.5mmTRS   using D-sub’s pins 2, 3 and 5 to TRS respectively.