Asterisk Audiocode siemens issue

Hi,
I am having audiocode MP124 and i have given the proxy as my asterisk server, when i tried calling from asterisk server console it is working fine, if i tried from the pstn phone that is connected with siemens hipath 3800 pbx it is not working, i cant able to make calls, i am using nat ip in my asterisk server to reach the outside sip server,

pls help me…

thanks