Use the link on the lefthand side of the user page to download the softphone.
The page should recognize your operating system and all you have to do is click next. If it doesn't just select your operating system and click next.
Select the community or free version of the software. The system has all the functionality that you need.
Once the download has completed, install the software on your machine. Please note that you will need administrative privileges to do this.
Once it is installed the Softphone client should look something like this.
You can check if your softphone has configured properly by going to the Preferences section within Settings and checking that these settings have entered properly.