Helper assignment has been turned off for security reasons, use the iptables CT target to attach helpers

I have this problem with SIP from outside of the network (over OpenVPN Connection) ...
I installed already the iptables-mod-conntrack-extra package but i still get this message (and no voice over SIP).

Which additional modification is required to get this working?
I saw this thread but i have no clue about that modifications:

you probably want a proxy

Hm dont know if i need that ... is there no "simple" way to pass through all voip traffic through the firewall/vpn/nat?