(519) 763-5000 is the local access number in Guelph for Distributel.  If a person uses Distributel as their LD provider, they dial this number to access the system and call other people.  On the receiving end of things, you will see this number whenever anyone using Distributel calls you using the Distributel LD service if you are in the Guelph area.  In Toronto, you would see (416) 969-5000, in Ottawa (613) 237-7000, and so on.  As for the dial-tone on call-back, that's how their system works.  When a subscriber dials the local access number it rings through to the system and gives them a 2nd dial-tone at which time they enter the desired LD number.

called cell phone left no message

How do you find out the number of that person that called you from that number then?

I to just received a call from (519) 763-5000 and when answered just hung up.  After researching the number, it appears to belong to Distributel

Received this call tonight...did not pick up, did not leave a message.
Ontario 9:10 pm

Received this call tonight...did not pick up, did not leave a message. Ontario 9:10 pm