Hello Im Sourish, I'm a Linux, asterisk, network expert. I can do this for you within few hour. I would install OpenVPN with a web portal on your hub server. I will install standard OpenVPN service and opensource web portal tomanage the openvpn. You would be easily able to create and download user configuration and start using it. Will install vpn client applicaion on the debian server and import the server config file. Please let me know and we can talk further on it.
You can try it with me and if you are satisfied then you can pay me. Looking forward to hear from you soon and hope to be a part of this project.
Thanks
Sourish