VPNs route traffic through an encrypted tunnel, introducing latency that degrades call quality and may result in choppy audio or dropped calls.

The process of VPN tunneling and encryption adds overhead, slowing down connections. This reduced speed poses challenges for VoIP systems in transmitting voice packets quickly enough for natural conversations.

VPNs may decrease available bandwidth, particularly on crowded or slower networks. The quality of VoIP calls relies heavily on maintaining consistent, high-bandwidth connections.

Due to the limited number of VPN connection points, distant endpoints can lead to extended round-trip times, causing disruptions in calls.

VPN encryption obscures the original source IP address information crucial for VoIP call setup and quality optimization, potentially resulting in call failures.

Certain VPN protocols, like GRE, are not conducive to VoIP, causing significant degradation in voice quality.

Instabilities in VPN tunnels contribute to increased jitter and packet loss, significantly impacting the overall quality of VoIP calls.