SIP Cannot Answer

Recently I rent a Linux Virtual Private Server and I install asterisk on it. I transfer all settings of my real server to VPS but my sip account cannot answer the call!!
My SIP account is registered on SIP provider correctly and I can see it on “sip show registry” and “sip show peers” but the ANSWER is not work! When I have an incoming call on the SIP account for example the “Wait” application works correctly, but when the “Answer” application cannot run and the call hanged up!! I cannot have outgoing call too. When I execute the “Dial” application on SIP account I get a “Service Unavailable” response back from the sip provider!!
All settings of VPS is the same as my real Server but it does not work!
I have exactly the same problem with virtual machine of Virtual Box!

share your dialplan and sip.conf