Copy the file openser.nat (http://www.asteriskguide.com/openser/openser.nat) to /etc/openser.cfg. Let's analyze the required changes to the openser.cfg file.
NatHelper and MediaProxy are responsible for handling the NAT traversal. Some functions such as fix_nated_contact(), fix_nated_register(), fix_nated_sdp(), and nat_uac_test() are made available by the NatHelper module. The MediaProxy module exports the functions client_nat_test, fix_contact(), use_media_proxy(), and end_media_session().
loadmodule "/usr/lib/openser/modules/nathelper.so" loadmodule "/usr/lib/openser/modules/mediaproxy.so"
The modules NatHelper and MediaProxy comes from two different solutions (RTPproxy and MediaProxy). There are some redundancies between them. So we will disable some functions from NatHelper such as natping. We don't need two natping processes to be active simultaneously.
modparam("nathelper", "rtpproxy_disable", 1)
modparam("nathelper", "natping_interval...