Once you have downloaded and started to run the installer for your Express Talk Softphone there will be a number of screens to go through in order to fully install the program.

Please make sure that you deselect all recommended extra programs during the set up.

There is an option during set up to state that you already have a SIP account. Click this radio button as shown below and continue the installation.
The next screen will give you the option to input the details of your SIP account

Enter the Username, Password and domain as provided when you signed up with A2 VoIP and you will be able to make and receive calls once you complete the installation of the phone.