Is VoLTE not supported by default on GOS?

Yes and no. e.g. see https://grapheneos.org/usage#carrier-functionality

Wi-Fi Calling, VoLTE, Visual Voicemail, MMS, SMS, Calling and 5G (SA and NSA) all are supported, however some functionality may not be usable due to Google not supporting carriers on the stock OS officially or due to GrapheneOS not shipping proprietary apps required in order for this functionality to work on some carriers.

Generally 5G, SMS, MMS, Calls and VoLTE will work fine on GrapheneOS with officially supported carriers by Google.

Some carriers may restrict functionality, such as VoLTE, on imported Pixel devices as they only whitelist the IMEI ranges of Pixel device SKUs which were sold locally.

And https://grapheneos.org/usage#lte-only-mode

VoLTE / VoWi-Fi works on GrapheneOS for most carriers unless they restrict it to carrier phones.

So… it depends

If your carrier is official supported by google, your carrier itself supports it and your device is not imported, it is likely to work.