Scarcechicken Are Pixels officially sold in your country (Google Store)? Are they listed on the carrier's supported devices list? Does VoLTE and 5G work?
Also, try searching for generic tips about WiFi Calling (also called VoWiFi) for Pixels in your language, maybe someone has already solved this on a local forum, for example using APNs (Access Point Names, which GrapheneOS forces to be available and editable regardless of carrier settings).
There is also another solution, https://github.com/kyujin-cho/pixel-volte-patch/blob/main/README.en.md . It forces WiFi Calling, VoLTE and others to be enabled regardless of carrier support. Its effects are cleared after any system update, so you'll need to re-apply it often. It unfortunately requires Shizuku, which needs ADB debugging enabled (which, as some say, is a security hole). So, if going this route make sure to disable USB Debugging and Wireless Debugging right after you're done.
I'm fro Poland and also had issues with all 3 (VoLTE, WiFi Calling, 5G) and after some time (OS updates) they just started working one day. Now Pixels are officially supported here, because they started being officially sold about a month ago.