I've just been bitten by this, but it looks like Google blocks VoLTE and 5G in 'unsupported' countries [1][2][3], i.e. countries where Google doesn't 'officially' sell Pixels (whatever that means). So even though your operator and SIM support VoLTE, it might be artificially blocked by Google. This seems to be a know issue for a while now and can be resolved by rooting and enabling VoLTE/5G through modem configs, carrier settings, etc. [4][5]I know it's futile, but is there any way to get Google to address this?
[1] https://support.google.com/pixelphone/thread/131223790?hl=en
[2] https://support.google.com/pixelphone/thread/78464113/pixel-volte-and-5g-support?hl=en
[3] https://support.google.com/pixelphone/search?q=enable+volte&from_promoted_search=true
[4] https://github.com/swangjang/Pixel-Redbull-Carrier-Patch
[5] https://github.com/stanislawrogasik/Pixel5-VoLTE-VoWiFi