GrapheneOS aims to work with all carriers that are officially supported by Google on the stock operating system on Pixel devices. 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. GrapheneOS extracts CarrierConfigs, APNs, modem configurations, Visual Voicemail configurations and MMS configurations from the stock operating system to ensure users get easy carrier support that "just works".
Please note that in some regions, LTE is referred to as 4G.
Generally 5G, SMS, MMS, Calls and VoLTE will work fine on GrapheneOS with officially supported carriers by Google. Wi-Fi calling may vary due to a reliance on proprietary Google apps which GrapheneOS does not ship.
https://grapheneos.org/usage#carrier-functionality