Help on "Call failed: Declined" error!


I’m new to this community & asterisk and this is the very first asterisk pbx server that i’d ever setup. Now i’m stuck at this & still can’t find out a way to solve it.

I’m using Asterisk for SIP VoIP server & eyeBeam as client’s softphone. After the installation, everything gone fine with default config. I can make call to another softphone in the LAN as well as outside network. But when i try to delete default extensions & add new extension (modify extensions.conf & sip.conf), something went wrong & every existing extensions stop working even if i restore deleted extensions!? server keeps response with “Call failed: Declined” error. I try to read manual & did search on internet but still can’t figure out what happen. Maybe i miss configure some exten or what? Can anybody help me figure it out, please?

Thank in advance.