You can simply use the callback function in the login area. To use this just click on "Callback" in the left menu.

Then you will see the form as shown above. There you can enter your phone number in the first text field.

In the second text field you can enter the phone number where you wish to call.

Now you can click on "place calls" and the system will call you back on your phone number.

After picking up the phone, the system will place the call to your partner.