"some people even have home VOIP systems"
Here many ADSL and almost all consumer FTTC (and obviously FTTH) connections wholly remove the POTS analog line and force user to use only VoIP for fixed phone calls, so it's a lot of users.
I do not know if consumer router with built-in ATAs use ALG to handle VoIP protocols, or use other methods. ALG is not strictly required - it is possibile to explicitly open and map the required ports, or use STUN and NAT keepalive to let the VoIP proxies make inbound calls. Users of external ATAs or small VoIP gateways/PBX may not know what method their router is using.