Voice over IP (VoIP) is commonly known as phone service over the Internet. Any service using public IP network requires certain extent of security. Demand for security in VoIP technology is increasing. VPN is one of the commonly used methods to secure VoIP traffic. In this paper, the authors simulated behavior of a VoIP communication while running over a GRE VPN tunnel using OPNET modeler 17.5. During the simulation, such performance parameters as: choice of voice signaling protocol, voice codec, parameters arising from network QoS (in this paper, homogenous vs. heterogeneous network environment) and type of VPN tunneling protocol, were examined.