I have switched over to Optus and I'm able make and receive calls but the phone does not connect to the 4G network. I have updated the access point names and the phone says it's registered on the LTE YES OPTUS network. The phone is a Sony Xperia
Check the phone settings aren't set to 3g only. Try a mates SIM and see if you can get 4g. Try your SIM in the mattress phone and see if it can get 4g. Perhaps drop into an optus store and get the SIM replaced on the spot for free.