vicidial.org

I have installed and configured vicibox server.

When the agent logs in with the X-lite sip-softphone, it rings and is connected to the Agent interface to make calls.

But when an Agent makes a call, He hears a voice saying -
" I am sorry. That is not a valid extension, please try again.", while I can directly log in with the X-lite sip-softphone into asterisk server and make calls.

I have removed all dial prefixes in vicidial configuration interface. And the number is dialled correctly as can be seen from the agent reports.

I am unable to find the source of the problem.

Some asterisk CLI output would help, along with your dialplan settings in admin interface.
Most likely there is misconfiguration in your dialplan. You may also search the forum as similar questions have been asked and answered many times.