Help Overhead Paging Interface (Viking FXI-1)

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I'm not entirely sure if this is the right forum for this question, so apologies if it's out of place. It does relate to audio wiring though, so hopefully your knowledge in this arena could be of help even if it's a different application. It's worth a shot anyway! Thanks in advance for any help or insight you can provide.

Here's the issue I'm having:

(Please note that this isn't my field of expertise so my knowledge of terminology is probably not the most accurate or technical -- hopefully I get close enough so you at least know what I'm referring to :) )

I have a client I do IT work for who's getting a new VoIP phone system. Normally this is an easy project, but I'm having real problems with getting overhead the paging system to work with the new equipment. For reference, here is how the old system works:

There is an AT&T device on the network that is part of the current VoIP system that has a 3.5mm aux out. SEE PIC.

This aux output connects to INPUT B on a Speakercraft auto-sensing switch with a "3.5mm to RCA" cable. INPUT A comes from a Sonos Connect using standard RCA cables, and the switch outputs to their amp using standard RCA cables. Music plays by default via INPUT A and INPUT B is "switched" when the paging system is activated. When the page is over, the switch cuts back to INPUT A and the music continues. All works as it should. SEE PIC.

Unfortunately the new VoIP system does not have a convenient aux out for paging. So for this setup we are going to be using a Viking FXI-1 to bridge from the phone system to the audio system for paging. SEE PRODUCT MANUAL.

While we're confident in going from the VoIP system into the Viking for the audio source, the unknown was going from the Viking into their existing setup. Rather than wait until the day of install and assume it would work, I created a test setup by connecting a traditional analogue phone into the Viking input for the paging audio source (terminals 5 and 4). If I could verify that the output worked, we would be in business.

The initial plan was to take a pair of RCA cables, strip one end, connect that to terminals 6 and 7 on the Viking, then plug the other two into INPUT B on the switch. SEE PIC. Nice and simple. You pick up the phone, the Viking kicks on, outputting the audio. The switch senses that signal, cuts from A to B and the paging is a go. Unfortunately, this did not work as planned. For whatever reason, the switch refused to sense the signal on B and cut over.

To rule out a few things, I first tried connecting the Viking directly to the amp (bypassing the switch). Sure enough, this worked without issue. Speaking through the phone would play through the amp/speakers. So I know the Viking was outputting the audio properly. To rule out the switch being the problem, I tried a different auto switch (an MCM Electronics 50-8396). Same problem. The switch would not trigger. I also confirmed that the sensitivity was turned up to its highest setting and the Viking volume was at max. No effect. One really strange thing I noticed was that if I took a screwdriver and pressed it to the screw on terminal 6 or 7 with this set up, the switch would trigger and cut over to B.

At this point, I tried calling Viking and they said the setup should function as planned and they had no idea why it wasn't working. Of course, they said it must be a problem with the switch (to which I reminded them that I've tried two entirely different switches). No help there. They then recommended I not use the auto switch and that instead I use the Viking's built in switching capability meant specifically for overhead music and paging. I figured I'd give that a whirl since I had nothing to lose. If it worked, great. If it didn't, Viking couldn't blame it on the switch.

So using the diagram for set up A in the Viking manual (page 2), I re-wired things accordingly. Sonos red RCA output > Terminal 10. Terminal 9 > red amp input. Terminals 8 and 7 connected in a closed loop. Sonos white RCA output > Terminal 6 > white amp input. SEE PIC.

This set up actually worked. Sort of. The music would play, and when I picked up the phone, the music stopped and cut over to the phone/paging audio. The problem is, the music sounded TERRIBLE. Distorted, faint, and warbly. Oddly enough, I discovered that I could fix the music quality issue by letting the shield wire from the Sonos white output touch the shield wire on the paging amp white input wire (see arrow in photo above). When this happens, the music would sound great, but then the switching didn't work. I'd pick up the phone and could hear my voice but it was playing WITH the music -- not cutting over.

I'm at a loss here. I reached back out to Viking, but my feeling is that they're going to tell me that "it should work" and that's about it. I'm hoping someone here can see the obvious flaw in what I'm doing or can make some sense of some of it. From what I can gather, the shielding wire in the RCA cable clearly performs some sort of important function but I'm not sure where it's supposed to go or what to do with it on the Viking. As you can see in the photos above and this photo, when I strip the end of the RCA cable, I've just been wrapping the shield wire around the base to get it out of the way.

Any help would be greatly appreciated. And sorry for the long post. I wanted to get everything in there.
 
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