Configure Asterisk on Public IP

i need Help to configure My Asterisk server on Public IP to Access it from Public Network .
I done NAT to the Local IP And am able to Access FPBX GUI from Public Network but still am not able to access to Extinction from any software Like ZOIPER.

You need to configure one or more of the options that tell your channel technology driver its public address, or how to find it, and which sun-networks are on the private network.

Also make sure that you have port forwarding for both your SIP signalling and the RTP media streams.

If may also be necessary to set some options controlling the use of rport parameters and comedia, although, don’t confuse this with teh primary need to tell Asterisk about its public address, even if chan_sip names this parameter in a way that makes people think it is the primary parameter for NAT (it’s mainly for the opposite case, when the phone is inside, and Asterisk is outside).

Details of these parameters can be found in the sample configuration files for your channel technology driver. Unfortunately, these do not seem to be included in FreePBX, so you will need to obtain them from the source tarballs.

There may be ways of setting these using the FreePBX GUI, but this is the wrong place to ask about this GUI.

FInally note that you will receive toll fraud attempts within minutes of opening SIP to the outside world.

Thanks for your replay ,
Actually am not an expert , so if you can guide me with steps will be thankful .

Consider to add security system to prevent attacks.