Have you signed up with a VoIP service provider?
Express Talk is kind of like your ordinary telephone. It will let you call other destinations if you plug it into an active and paid for phone line from your telephone company, but it's just a dead rock on its own. So in order for Express talk to actually phone other numbers, it needs to be set-up with a VoIP service provider.
Here is a list of recommended VoIP services for Express Talk
http://nch.com.au/talk/sip.html
Most VoIP services will let you make in-network calls to other users using the same VoIP service for free and will charge low rates for actual PSTN (your regular analogue service) destinations around the world.
To give you an idea of how cheap the rates are, there are some providers that offer unlimited calls to North America for just $20 a month, and other services that charge per minute at only 1c/min to destinations in North America at no monthly rate. Most providers also offer incoming phone numbers (DID) that go directly to your VoIP account. You can usually purchase phone numbers from anywhere in the world, not just limited to your local area. This is great for example if you have your parents in another state and want to let them call you at local rates using their regular telephone service. Incoming numbers vary significantly in cost, I have one that costs only $1/month for unlimited incoming, but others cost up to around $8/month.