First, sign into the Grasshopper app with the same username and password you use to log in online:
Next, enter your iPhone's phone number to register your phone:
Finally, select your outgoing number. If you have multiple Grasshopper numbers (e.g., a toll free and local) you can choose which number will show up as your caller ID when you call others from the app. If you only have one Grasshopper number, it will automatically be selected for you.