You posted Vici version but not the FULL version with build. You also should post your installation method (with a link if it's not Goautodial from .iso). And for the record, it's not been "astguiclient" for over a decade. LOL: It's Vicidial.
All that being said:
Extension got registered successfully, even we got a message on Vicidial that extension called but nothing on asterisk CLI.
Extensions don't register. SIP accounts register. So do IAX2 accounts.
Please provide the basic information for the SIP or IAX2 Account in question. And what (specifically) you've put into the Real Time field. Note that the extension (ccxxx?) and context entries may be useful.
You could also try to turn on debug in asterisk cli. It may provide some insight. Some users include an "s" extension in their [default] context to capture invalid dials, but it should notify you of those anyway.
Then there's the API log which may bear fruit, since I believe this is initiated by an API call.