Vpn

hi my dear all numbers. i am new in here. so if i do any mistake plz forgive me. i want help from all.
i install freepbx in my Ubuntu 12.04 lts desktop version. everything is ok.but the problem is i can setup vpn with free pbx. as because i want to terminate calls for voip. i have a soft-switch. i have no real ip address in my freebpx. so i need vpn for ip address for add ip with my switch. but i cant understand how i do it. can anyone plz help me.