Why VoIP works without problems?

ER-X 1.9.0

I know there is a  SIP "helper", but what does it do?

For security reasons I use on my two phones different SIP ports (11111 and 22222). With my old router I had to forward this ports and to open the firewall.

Behind ER (default firewall) both phones just working without further configuration. Ports 5060, 11111 and 22222 are stealth (grc.com). How can I be called without open ports?

Please give me a short information.