internal SIP phones and external sip server

The problem is that without sip helpers voice goes in two directions, but additional board of phones doesn’t show anything.
If I activate helpers, boards work fine, but voice goes just in one direction on outgoing calls.
As for NAT, i masquerade my internal IPs as one of my external IP.

What should i add to Filter rules and maybe to mangle to use the full power of my phones?

Big thnks for answers.
Sorry for my English. =)