Many routers have the SIP ALG setting, which inspects VoIP traffic packets, enabled. They also often come with SPI and SIP_TCP enabled. When these settings are implemented incorrectly, they can modify VoIP traffic in unexpected ways that corrupt the packets and make them unreadable. This may result in unexpected behavior such as phones not registering and incoming calls failing.

To address this problem, check your router’s settings to ensure that SIP ALG, SPI, and SIP_TCP are turned off.