First, click the dropdown at the top. There you will choose from one of your LiveAnswer scripts.
Then dial the number you'd like to call. Once you hit CALL, it will dial the number you input and it will show the number you in step 1 as your caller ID and not your mobile phone number.
If you click on PROFILE and turn on Allow VoIP, then the LiveAnswer app will make calls over wifi, when available, rather than using your cellular connection. That will save you on data usage.