SIP ALG can be both useful and a pain.
This can become apparent when trying to setup a SIP connection to a PBX on the internet somewhere. Or alternatively, dialling into a local PBX from externally.
Many do no realise that SIP ALG is enabled by default on most routers. It is a NAT tool that inspects SIP Messages and transforms the Private IP and Ports to Public IP Addresses and Ports.
Leaving it enabled can cause issues such as one way speech or calls just dropping out unexpectedly.
This is how you turn it off on a Cisco IOS router:
R1(config)# no ip nat service sip udp port 5060
Port 5060 is the standard UDP port for IP phone communication, this may differ in your environment.