I talked to my operator, Telia, here in Sweden and I got this answer:
I do have VoLTE support from my operator, as well as calling over WiFi. But the quality is still quite poor. So not sure if this is really to blame.
I haven’t tested the voice/video call quality with this latest update though.
I would def #contactsupport to ask if they can add your provider. I assume there are many out there and FP might not think about all of them from beginning. Sweden is the first country in the EU I’m aware of that will shut down both 2G and 3G, most countries keep one as a fallback for calls when VoLTE is not available/possible. Shutting down both, means all older Smartphones will become useless…
It might be a combination of things. When I call my own FP4 with an iPhone 13, the quality is super bad through the iPhone 13. I then hear a choppy robotic voice coming from the FP4, it’s almost incomprehensible. But when I call my own number with my FP4 and leave a voicemail message, the quality is okay when I replay it. It’s not the crispy iPhone quality, but it’s definitely not what I hear when I call my own FP4 with an iPhone either. Maybe the iPhone 13 is the problem, I’ll check that sometime later. I also tested with Nokia’s and Samsung phones in the past, people then complained about poor quality. I haven’t called myself with those phones.
For now I’ll mark this issue as resolved. Since Signal is indeed much better now with voice messages and people say the quality with video calls is okay as well.
I have now updated my FP4 and my fathers FP4. It’s better now but not ok. When we have the FP4 close to the mouth its ok but as soon as you move the phone away about 10 cm from the mouth the robotic sound is back. I tried to call both from my own FP4, a Samsung S20 and from my operators softphone on my computer to my fathers FP4 with the same result. Hope they will fix it. I didn’t have any problems with my old FP3, that one worked fine both with cellular and VoLTE.
By calling myself and leave a voicemail I was able to do more tests without bothering other people.
The robotic voice with choppy audio starts when blocking the side microphone. I usually don’t block this mic myself. But when I call myself with an iPhone, the other caller holding my FP4 likely does this every time. Maybe you block one of the 3 mics as well?
The mics are positioned poorly, and in no way people should be taught how to hold their phones. That sounds a bit like the antenna issues of a certain iPhone release. But at least if we can identify the issue and reproduce the fix, we can move forward in some degree.
It for sure fixed it and improved also the Assistant. I would say it works more reliable that my Pixel 3 (or this is due to recent Google software improvements). This one can be crossed off the issue list.
Very good guess: I just disabled VoLTE and my call voice problems disappeared.
To do so, OMG, please search around in the so many Android-Setup-Flavours.
(Google: Pleeese do once some very basic work to streamline setting-menu structure to keep the stable at least some of the next releases!)
I experience this issue now with Microsoft Teams, people can’t hear me, they say the quality is unaudiable. Will report to FP support.
Oof, just when I thought all is well… Is it just like the previous Signal issue, where it doesn’t matter whether you use a headset or not?
Anyways, Signal voice messages are now working well for me, too! Yay!
Using a Bluetooth headset fixes it. But make sure you have them with you, otherwise your colleagues will not be able to hear you 
When I call someone, the other side often complains that they cannot hear me. They hear an echo or a very unclear voice. In the beginning I thought that I put my finger on the microphone, but that is not the case.
When I use a sim card from another provider, the problem stays the same.
I bought my Fairphone 4 some 6 weeks ago.
Using the sound recorder produces a recording that sounds like I’m talking through some old cans.
Anyone an idea how to solve this?
I have the exact same problem. People always have complained about my microphone but since a few weeks it got worse. I really need a solution for this.
Welcome to the forum @Jaapd and @Decide1000
You may want to read up on this/these issues
I suggest you start here as you note your queries have been moved to an existing topic that may help.
and you can use the search function etc. for echo, low sound etc.
Try if it improves when you remove permanent access of the Google App to the microphone,
I have the E.foundation software on my phone and disabled access to the microphone from all the apps (except the telephone of course). Hardly no improvement (after one call).
The receiver of my call thinks it’s probably the noise canceling that don’t works OK.
Ok with e/OS its another “story” I would check the Murena Forum and probably open an issue there as it might be a Software issue, rather than a Hardware bug
So does this mean this cant get fixed? In my case, even when i’m not covering the mic, the person im calling has lots of complaints… The audio input is very inconsistent. It seems that only the left sided microphone works when you’re using speaker mode
I just now called myself with speaker phone and regular setup. Both sound mostly the same. Still no iPhone quality, but also not inaudible. I’d suggest you call yourself as well, leave a voice message, experiment a bit and then forward that information to FP support.
It may be a carrier issue, which carrier do you use and in which country? When you #contactsupport, make sure you relay this information. They can then work with the carrier to improve the call quality.