Re: "some people even have home VOIP systems"
Note also that SIP ALG can cause problems to VOIP systems too. I need to turn it off on my routers for example otherwise our cloud PBX's packets get mangled and it does some weird stuff.
For example, outgoing or incoming calls are fine but forwarding the call to any other number will result in the internal caller and the caller being forwarded to being unable to speak to one another. When the call is actually put through the external and internal users can talk again.