No Outbound Calls

[OSCS]

I have one extension that cannot make any outbound calls.
No outbound internal nor external calls can be made from this extension.
I have checked the affected extension's settings and compared them with working extensions, and they are all exactly the same.
Please help!!

 

[leejor]

What does the 3CX activity Log, of a failed call show?
Is the call making it to 3CX?
Is this extension local, using STUN, behind an SBC?
Does the set show as registered?
Is the IP blacklisted?
Can the extension receive calls?