Electronic – Transmitting audio over variable voltage DC power line

Pretty much every major (and minor) vendor of network testing hardware has products for something called VOIP (quality) (passive) monitoring/analytics or thereabout. These compute aggregate statistics on the quality of the conversations, among other things. These usually don't run from your cell phone, but from a specialized routing/testing device. There's an ITU standard for VOIP passive monitoring called P.563 (this is the recommendation number) and also called 3SQM (the name of the algorithm), but it is not (yet) widely used by testing equipment manufacturers, and has been the subject of some criticism, e.g. [1][2]. So it's a bit of a wild west out there with respect to product features etc. for passive monitoring of VOIP quality.

I'm not endorsing any of these products, but here some examples:

• http://www.gl.com/packetprobe.html
• https://www.netiq.com/solutions/unified-communications-voip/voip-quality.html
• https://www.sevone.com/supported-technologies/voip-monitoring
• http://www.wildpackets.com/products/omnipeek_network_analyzer/voip_monitoring

You might be able to find some free software that does something similar, although you'd still need trunk-router-grade hardware to run it on. I was only able to turn up some 3SQM non-free software, which requires conversations to have been recorded by other means. There is a free piece of software called http://sourceforge.net/projects/voipmonitor/ but this does not do conversation-level quality analysis, but only testing for bulk suitability of the IP link to carry VOIP (ITU G.107). But you can probably at least use it to record conversations.

ITU has also put out a VOIP active conversation quality testing standard called PESQ and this is widely used by (active) testing products, but these inject a reference signal at one end and you don't seem to want that.

So the above generally covers "give me a way for me to automatically test a phone line audio quality". However, for your specific problem "the audio quality suddenly deteriorates and the outside party cannot hear us unless we talk very loudly", the issue may not even be (automatically) testable only from your side... If the packets leave your site unmolested but get messed up (volume-wise) by your provider/carrier or by some of their peers (probably not all), there would no way to establish that by just looking at packets on your end. You'd have to record/monitor the conversation from the other end to apply 3SQM to it.

My experience with cheap VOIP providers is that when they have (intermittent) quality problems the contracts/SLAs are harder to enforce than they are to terminate.

There's some lovely 60s brushed-aluminium-and-wood casework there! And quite a decent job by whoever renovated it, and did you the favour of labelling the wiring on the inside of the case.

You might get a better reception on /r/electronics than here, due to the open ended nature of the question. Step 1 has to be to reverse-engineer the original system, either by finding its original service manuals on the internet or by drawing a circuit diagram from the single-sided PCB. It's quite clear from just looking at it that all those unused screw terminals connect to the four-position radio channel switch. It looks like there is no local tuner as you say, the switch selects an audio signal from the screw terminals. Whether it amplifies it locally is unclear: we could do with a photo of the other side of the PCB, which will involve taking the knobs off (grub screw visible at top) and undoing the nuts on the shafts. I suspect (given your comment about power) it doesn't amplify it locally, or the amplification is only for the microphone (which I can't locate visually).