Why I can hear ringing tone even a SIP (Softphone) are inactive?

I tried to dial the DID number(Direct to Softphone) from my mobile phone. The softphone is not active (PC is off), but I still can hear a ringing tone from my mobile. Does anybody know what the settings I need to change to fix the problem? Isn’t because Call Waiting is enable?

Sounds like either your DID is not setup correctly, or FreePBX still think the softphone is registered (unlikely).
What do your logs say ?