The person at extension -five- is on the phone.

Does anyone in Canada, or Ontario, get calls from Iristel numbers, that you don't answer, but if you call back, you get "The person at extension - five - is on the phone.  Please leave a message after the tone".  Naturally I don't leave a message, but I want to know who this person is at ext. five.  It's always extension 5.  The last example of this was from 6838018582.

That is a very common recording.  It isn't necessarily associated with any particular number.  That said, it would be more helpful if you would post your comment/question under the number that called you.  If the thread doesn't already exist, you can start it.

Pudge is right, you would be better off reposting your message in the thread of the number(s) that called you.
The list of "Latest Forum Threads" is limited to just a few entries and newer/updated threads push older ones off the list.  This Forum thread will disappear in a day or two.
This site is based on the concept of searching for the number in question and posting in the thread that is returned.  Even if nobody has posted before a number thread will be presented for posting in.  Only 10-digit numbers are recognized.

The thread for the number 683-801-8582 you listed is currently empty, which means that if anyone else got a call displaying this number (which may very well have been "spoofed"), they did not bother to look it up on this site.
A web search for the number turns up nothing but links to fake number look-up sites that will ask for a credit card in return for dubious (at best) information.  There are no references to this number belonging to a business or a person.  However, that does not mean that it is not used by some business, just that this number is not on their website (web search engines can only report on what their "spiders" find when crawling through webpages).

While the response you heard when calling back is interesting, it is hardly anything to worry about.  If a call is truly important, they should leave a message if they do not get an answer.  You also have to consider that the caller may have "spoofed"  the number on your Caller ID, in which case the number you list has nothing to do with whomever actually called you.

Extension number 5 may be the default value programmed into the voicemail system that answered your call.  The person who set it up may not have bothered to read the manual, assuming it was plug-and-play.