Definitely not a collection agency because I don't owe money. 3 Calls received from that number over the weekend and I sent them all to VM but no message was left.

Something wrong with your account?  A likely story.  More likely somebody trying to phish you via Skype phone so the call's untraceable.

I am at work, I cannot be accepting calls while at work, I did not answer it, I called it back on my break and it was " the number is not in service "
this is really bothering me. If it was a fax calling, wouldn't it have handshake or fax sounds. ?

....which makes me wonder how this can be done?
A phone number calls my line, and when you call back, you get a "not in service" message?

On October 24, just before 2 in the afternoon, I received a call from this number on my blackberry.  Did not answer; they did not leave a message.

I received this call on Friday October 3rd 2008 at 6:54pmFriday October 3rd 2008 at 8:42pmSaturday October 4th 2008 at 10:35amandSunday October 5th 2008 at 4:47pmThe calls on October 3rd came across as faxes,but the calls on the 4th and 5th left no message or fax sound.Caller ID: (613) 232-7437