WhatsApp audio calls fail to connect

I recently bought a second-hand Nexus 6 running Lineage OS 18.1. Everything working great with one exception: WhatsApp reproducibly fails to establish audio calls with the error: > Couldn't place a call. Make sure your phone has an Internet connection and try again. I tried it from two WiFi networks, where other phones work without issues, so it cannot be a network-related problem. Also, the Nexus 6 does have an internet connection, even WhatsApp itself can send and receive text messages without problems. Since Lineage OS is not officially supported, I installed a factory image (Android 7.1.1, the latest available one), installed WhatsApp there as the only app, and gave it all possible permissions, and still the problem occurs. Interestingly, other messengers like Telegram have no issues placing audio calls, just WhatsApp does (unable to place or receive calls). I tried to contact WhatsApp support, but: - Through the WhatsApp chat, I only got some machine-generated replies which didn't solve the issue, then the chat has been closed. - Through the contact form on their website, I got an email reply, but I couldn't answer the email as the email I sent didn't go through. I received an automated email with > 554 5.7.1 POL-P6 Message refused https://www.facebook.com/postmaster/response_codes?ip=209.85.160.45#POL-P6 - I answered via the contact form again, but at some point, the "submit" stopped working. When clicking it, just nothing happens anymore. So now I'm still here with this problem and don't know what to do next.