(question) hello sms. Various questions topic

Good afternoon guys. I came across an interesting sms app yesterday and decided to give it a try. It's hello sms. I loved the design, simplicity and very easy to use. However I was really confused with some aspects of the app.

1). It says it's an sms app, but it's often compared to a messing app like whatsapp. I was able to send sms, but it says that it's free of charge if the other person has hello sms as well. So if the other person doesn't have it, what's the charge per sms? Would I see some type of fees on my Sprint phone bill? The developers don't provide much info and description on the play store. I tried to contact the developers by email twice and no damn response. If someone is using this app please share your experience. And wether or not it's free or costs money.

Thanks