Question:  Can I interface a Clear-Com MS-232 party-line main station to a Telex BTR-800 wireless base station?  What cables do I need etc?

Answer:  Yes, the BTR-800 wireless intercom is fully capable of interfacing into both channels of a MS-232 main station. 
The connections from a MS-232 party-line main station to a BTR-800 are pretty straight forward to get both channels (A and B) up and running on the BTR-800. 
Hardware connections: 
You will need two 3-pin XLR audio patch cables (male XLR on one end, female XLR on the other end).  The cables attach between the MS-232 and the BTR-800, one cable for Ch A and one cable for Ch B.  If you have no spare Ch A or Ch B 3-pin XLR jacks on the back of the MS-232 then you will either need to construct or buy some SP-3 line splitters to get the extra connections on the MS-232.  Basically the SP-3 is a passive cable split.  The BTR-800 has 3-pin male and female loop thru connectors on the back of the BTR-800 for Ch A and Ch B.  Once the cables connections are done, the BTR-800 needs to be setup as follows:

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On the rear panel of the BTR-800 set the IC switch to “Clear-Com” mode.
On the front panel of the BTR-800 press the IC select button for the 2-wire / 4-wire IC on Ch A and Ch B to “2-wire” mode (green led for 2-wire should be “on”). 
On the front panel of the BTR-800 note the plastic slotted screwdriver adjustments for IC A and IC B input and output levels. 

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With a BTR-800 beltpack on and a headset attached, have the BTR-800 user talk / listen on Ch A, have another user get on the wired intercom and have them talk / listen on Ch A.  Adjust the IC input / output for Ch A on the BTR-800 to balance the incoming and outgoing IC audio levels between the two systems. 
Repeat the previous step for Ch B to complete the process.

Note:  Solution is also attached below as a PDF.