I’m currently overseas and have activated all relevant roaming.
I am able to use data and send text messages and receive phone calls however I am unable to make any calls
See if this still works:
You can call Optus for free on +61 2 8082 5678 from your Optus service to speak with our customer service teams between 8am and 7pm ADST.
Otherwise login to Optus LIVECHAT here online (24/7).
You could also install something like WhatsApp that will let you make a phone call using data for a few important calls? Your friend / family / business contact needs WhatsApp also installed (Ios or Android)
A couple of other things to check are that you are dialling the number in the correct format like in Peter's example with +61 to make a call to an Australian number.
If you have that right I suspect you have international call barring enabled on your service which you can chat to us about to have removed.