Please double check that the number you saved in the "My Mobile #" field is your actual Android device phone number. You can easily update the number you registered in the Settings section of the app:
If you're using Google Voice, you'll need to turn off dialing from Google Voice in order for dialing to work with Grasshopper.
If you're in Canada and you're using Telus, then the problem is that Telus won't properly show us your caller ID. In other words, instead of showing us 604-345-1234, they're showing us 604-345-0000. This makes it impossible to detect that it's you calling, and this is required for the app to work properly. Unfortunately there is no way around the problem and Telus has not fixed this in several years despite our many requests.