I like your app, BUT :
1. You have not done your homework on the market before creating it, if you want to monetize it you should have and you probably should have targeted Europe where it's legal to book a cab.
2. How do you convince your app users that it's better to use the app than to call? App is free, calling costs - may work, may not. App needs internet, calling does not. That's where marketing comes in.
Don't try to find guilty, try to find how to fix it.
Good Luck.