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SIP ALG (Application Layer Gateway) is a feature which is enabled by default in most Cisco routers running Cisco IOS software and inspects VoIP traffic as it passes through and modifies the messages on-the-fly.

For our Hosted VoIP service, including our Single User services, SIP ALG must be disabled on your Cisco router.

This article will show you how to 

• Configure your Cisco Router to disable the SIP ALG setting.

Log onto the router's terminal (command line interface) via telnet, SSH or serial console

enable
configure terminal
no ip nat service sip udp port 5060
no ip nat service sip tcp port 5060 (For TCP)