The phones use SIP as their signalling protocol. Many routers and firewalls have SIP specific settings that manipulate how SIP traffic is handled. This Application Layer Gateway’s intention is to help alleviate traditional issues that applications with specific addressing requirements like SIP have with NAT traversal. This can have other names (SIP Helper, SIP Inspection, SIP Transformations, etc.), but ultimately they all perform a similar function — identifying SIP packets and manipulating them. These settings almost always need to be turned off as they (somewhat ironically) will almost always break SIP.