Unable to connect another SIP phones

I am beginner. Just installed Asterisk now 1.6 and Freepbx configured extensions 5050 and 5051. When I call from either side, I get voice message: the number you have dialed is not in service, pls check the number and try again.

Pls help

My externs are registered. But i am not able to log in voicemail box.

I tried to reach voicemail box by softphone *98. It gives me same message : the number u have dialled does not exist, pls check the number and dial again.

Are the extens registered ?
Can you access the voicemail box for each extn ?