Softphone extensions make call but do not receive call!

Dear helper,

I have Asterisk 1.2.20; a pstn phone, an IP phone, and a softphone,
Wherever I call the Softphone it says User is busy, while I can call the other two extensions with this softphone!

The asterisk “Show Hints” command shows that the status of softphone extension is Unavailable! (I think it should be IDLE like the other two extenions.)

How come it’s possible and How can I troubleshoot?
(Yes i’ve checked other softphone-extension on other PCs, same result)

Thanks in advance,

Most likely a firewall issue on that specific computer.

Is “Do-Not-Disturb” set on the softphone that cannot receive calls?

Is the network reachable?

Do you have a firewall in the way?

Is the softphone able to register?