Make sure you have 4G calling enabled. Even though the handset would be using 4G/5G for data connections, it may fall back to 3G for call setup if this is not done. Once 3G has been shut down, calls would then not work.
You might also need to enable VoNR. Search for that in settings and enable it.
I have a Phone (1) on Belong (Telstra Wholesale network) and that is compatible with 4G Calling and VoNR enabled.
As per the Optus website:
You can also check whether your mobile phone is impacted at any time by texting the number ‘3’ to ‘3498’.
