I have tested ser with a T1 trunk GW not behind NAT, but using fixed IP. I then disable the use of rtpproxy and nathelper. However, when I use PSTN call to a SIP UA, only ringtones could be heard but no voice passing through it.

Does it mean that rtpproxy + nathelper must be used even in the case when UA and PSTN GW are not behind NAT/Firewall?