The guy is right. That happened to me once when I changed to a new phone. When you call Optus tell them about your new handset and they will need to activate volte for you. In the system, your old phones serial number is registered with Optus and when you transferred your sim to you new phone obviously with a different serial number and the system doesn’t recognise it.
So same thing happen with slow or unstable connection, usually they will ask you to reset the network setting then restart your phone to refresh setting and in some cases Optus will need to switch the service ON their end (depends on your handset and service)
Call Optus now.
I can always go in and check if the correct 4G APN has been flagged on our end.
Shoot through a PM and I'll get back to you as soon as I can. I need your full name, DOB and mobile number.