The prevalence and ease of packet sniffing and other techniques for capturing packets on an IP based network makes encryption a necessity for VOIP(Voice Over Internet Protocol). Security in VOIP is concerned both with protecting what a person says(by Encryption) as well as to whom the person is speaking(by Authentication). IPsec can be used to achieve both of these goals as long as it is applied with ESP(Encapsulating Security Payload) using the tunnel method. This secures the identities of both the endpoints and protects the voice data from prohibited users once packets leave the corporate intranet. The incorporation of IPsec with IPv4 increase the availability of encryption, VOIPsec (VOIP using IPsec) helps reduce the threat of man in the middle attacks, packet sniffers, and many types of voice traffic analysis. Combined with the firewall implementations, IPsec makes VOIP more secure than a standard phone line, In this paper the negative effect of adding security to VoIP networks has been measured for different simulation times using OPNET(Network Simulation Tool). the results show that transmitting voice over IPsec increase the end to end delay,delay variation(jitter),packet loss and call setup time.