Yes there is a cost associated in making the actual call to the phone. I am currently absorbing this cost to get initial users. The things we have to do to validate an idea :)
With the cost unpredictable, we can't say if it will be worth it. Plus with no price, it makes it seems like this is just a hobby and you may decide you're bored with in a couple of weeks.
To attract paying, commercial customers: state a price, and then give a free 3 month trial, no cc required.
A quick heads up on the cost, it will be 3 cents per minute to call an actual number from the website. If you had the app however ... this will be free.