Application-level gateway (ALG) is intended to assist users who have phones on private IPs. In many cases, it is implemented poorly and actually causes more problems than it solves. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behavior, such as phones not registering and incoming calls failing or even one-way audio.
Disabling SIP ALG varies, depending on your router. We recommend checking the manufacturer's handbook that came with your device. Alternatively, check the Internet and support forums for advice. You can enter your router make and model, along with Disable SIP ALG, into a search engine.