Emergency calls bypass the network restriction. I don't see it documented for GrapheneOS, but https://source.android.com/docs/security/features/cellular-s... states

'Android allows users to disable 2G at the radio hardware level on any device that implements the capability constant, "CAPABILITY_USES_ALLOWED_NETWORK_TYPES_BITMASK". This stops a device from scanning or connecting to 2G networks.

Note: Emergency calling is never impacted. A device still scans and connects to 2G networks for emergency services.'