Hi Kevin,
1. Yes we offer a mobile app for Iphone and Android.
Checkout the user documentation on how to use this feature:
Mobile App:
https://www.ringroost.com/user-manual.php#mobile-phone-app
PBX mobile control :
https://www.ringroost.com/user-manual.php#mobile-phone-control
2. Yes, Ringroost is a hosted PBX. So it entirely replaces an Asterisk based system.
3. We charge only for your usage, you can setup auto-recharge to ensure your account does not fall below a certain amount:
See pricing here:
https://www.ringroost.com/pricing.php