The PPTP is used at NAT/PAT environment as shown following figure. At this model, the
NAT/PAT router has public IP address at WAN side and private IP address at local side.
The VoIP gateway has not a public IP address but a private IP address. The VoIP
gateway communicates with PPP/PPTP server via NAT/PAT router and can get a public
IP address using PPP over PPTP tunnel. At this time, the public IP address of gateway is
not same address as NAT/PAT router. The PPTP connection is terminated at remote PPTP
server, across public networks.