1) This project contain 2 types of user . 20% user have public ip and rest 80 % client does not have any public ip
2) Most Default VPN ports are blocked
3) If the ISP can detect that all IP are traveling through the same port then port may be blocked.
4) As you know that VoIP calls first goes to VPN server and the server distributes the calls to clients. So if we use dynamic IP for client then there should be a mean that server should send packets the new path.
5)you can make a script which will do these automatically
6) when we used PPTP server that you helped me to configure you saw that i could not connect VPN from my local PC but you could use from your laptop.
7) So we should develop a VPN which will be unblocked in Bangladesh and VPN should be up 99.99%.
8) the VPN client will be in a linux PC. on the client LAN there may be 1-4 GSM gateways connected which will have private Static IP. So we should be able to access(Web Interface/Telnet) from remote place.
9) The VPN overload should be minimum.
10) there should be a of monitoring the VPN status from the Server. Like which VPN is established and What is their current IP. We are expecting a GUI for this.
11) There should be a way of accessing the remote client before even deploying the VPN. So that we can change required configuration in the client PC remotely(client have Private IP).
12) You have to give us detailed documentation about both client and server.
13) I will need customized ISO for client for linux/debian based os.
I think I've mention a lot many requirements. I have not told you which VPN should be used. Its upto use the best VPN you suggest.
Please suggest from your experience which will be good. Please let me know about your suggestion..I will need post configuration support for testing perpose.