You need to install some kind of outside line - either through VoIP or PSTN lines. If VoIP you will need a providor - I'd recommend on from the listed approved ones in the VoIP line install. The templates work well for each of those vendors.
If PSTN you will need a gateway (FXO). Again use a recommended one from the list.
Once one of those are done you can call people but it won't be free.
If you have setup a couple extensions you can call those for free. If you can setup an outside extension (with a friend) you can call them for free since they are just an extension of your system.
You can use another hardware phone or a VoIP software client for that.