Voip issue 18.x

[cyberbot]

Dear all.
Two ago we have updated two hardware to 18.12 from 17.9. after the update our VOIP Phone are not working proply.
the conversation drops like 2 second from time to time so the other side miss always 2 second from the conversation.
it kinda frustrating for the users. i've been reading on the forum and found somewhere that the frebsd 11/12 has changed the NAT / Firewall how it deal with UDP pakkets and we have to change the firewall from normal to Conservative which we did. unfortunately it didnt fixed it.
I dont know if i am the only one with this issue but i've reading that i have to do the next :

Disable source port rewriting
Set Conservative state table optimization
Disable scrub

Can someone please advise how to do so ? and if this the right configuration ?

Firewall WAN . no open ports
Firewall LAN   any to any