@Janie19: If you have Wi-Fi calling enabled, try heading to the Phone app > Tap the 3 dots in the top right > Settings > Wi-Fi Calling, and disable it temporarily to see if you're able to make a call via your network. Alternatively, toggle off Wi-Fi to see if this helps. Also, head to Supplementary Services > Call Barring > All Outgoing Calls, and make sure that this is toggled off. If you're on a PAYG contract, reach out to your network provider to see if you have sufficient credit to make outbound calls, and enquire about any restrictions on your account, or with the service in your area.
If you're still having issues, head to Settings > Connections > SIM Card manager, and ensure that Calls are assigned to your preferred SIM card.
