Calling Internationally on a Landline

I have an AT&T landline phone in Los Angeles, California, but I am not able to dial my son in Sydney, Australia. How do I get some international calling plan added to this phone? I expect to have to pay for this, but during this COVID-19 period, no AT&T personal are available to answer this question, and the automated chat feature  doesn't have a clue.  I only need international dialing on my landline, not on my mobile phone. I want to make calls to Australia, not usually get them