PWM whine near earpiece in low signal phone call and loud sound sent to the other end

[dsimic]

If the problem is a DC-DC converter inside the modem itself, there might be a hardware mitigation (beyond making the microphone lines more immune to pickup by caging or something similar) depending on how the noise gets into the microphone line (assuming it is that). Something a simple as a filter on the power connection to the modem itself might help. DC-DC converters send a lot of noise backwards to the source in addition to propogating it forward. Essentially, adding a line filter in a potential future 1.2c revision.

Please, keep in mind that introducing such changes to the main board of the PinePhone might pose a problem, unfortunately, because of the already passed CE certification. It costs a lot to have the phone re-certified, and stretching the terms of the CE certification might not be a good idea, especially because the changes would be "touching" the modem.

If the modem's DC-DC converter noise eventually turns out to be the root cause, it might be possible to have mitigation in software, by limiting the modem's transmit power, if that's possible at all (which probably isn't), and if it's possible to find the "sweet spot" that eliminates the backfeed of the noise while not adversely affecting the operation of the modem in poor-signal conditions.

After a bit of searching, it seems that limiting the transmit power of the modem is actually rather easily doable (woo-hoo!), by (ab)using the vendor-specific AT commands described in this official document.  Those AT commands look very interesting, and are not available in the standard official manual.   Testing out the effects of limiting the transmit power should be worth it, if you agree.

By the way, this document contains, among a lot of other stuff, the reference designs for the modem's power supply circuit.  Those might be usable.

As a note, configuring the required audio routing should be possible by using or modifying the audio configuration program that's available on Ondrej's web page.  I haven't researched that in detail, but I think it's rather easily doable once you get a grasp on the entire audio subsystem.  The hard part, dissection of the PinePhone's audio subsystem, has already been done by Ondrej.

That is what made me think it is doable and how to do it. Set things up like there with maybe a tweak or two to insert audio and start the call from mmcli with AT commands (can't use the call program since that would override the audio settings). But there will be quite a bit to learn in order to do it.

I might be wrong, but wouldn't passing "-2" as the only argument to Ondrej's audio setup utility activate the modem's digital audio interface, while keeping all other "usual" audio inputs and outputs disabled?  I might easily be wrong because I havent't researched the A64's built-in audio codec in detail, but the effects of the "-2" parameter might be exactly what you need.