Asterisk expert needed.

i have stuck on one problem everything is working fine, except one thing i want to use extension from remote internet i successfully register to my 3g mobile network to my office asterisk server, which is on my local office network and i can place call from my remote extension to my local extension. and voice and everything is working great but i can't initiate call from my local extension to my remote extension. Sip peers table output is given below......... Name/username Host Dyn Nat ACL Port Status callcentric/17772228417 [login to view URL] N 5080 Unmonitored 24 (Unspecified) D N A 5060 UNKNOWN 23 (Unspecified) D N A 5060 UNKNOWN 22/22 [login to view URL] D N A 31367 UNREACHABLE 21/21 [login to view URL] D N A 63916 OK (102 ms) 22 is my remote extension and 21 is my local extension. I can place call from 22 to 21 but i can't place call from 21 to 22 as my asterisk server says that my extension 22 is not reachable, though Zoiper sip client successfully register using extension 22 to my asterisk server. I know this is very simple problem. But as i new to this i can't figure it out. Budget $2-$5 as its a 2 minutes task.