I have one extension that can receive calls from other extensions, but cannot receive calls from outside. The extension for example is 521. If I dial 521 from another office phone, the extension will ring fine and user can answer. If I dial this number from my cell phone, all I hear is Extension Doesn't exists.

i am not sure, may be i need to configure the mailbox for this extension.

Please guide me how to do it.
Thanks in advance for your help.