P.S. You cannot call this number, it either doesnt go through or gives a extremely weird ringtone. Further supporting my theory that its a service number.

According to some quick research, I think this is from your service provider. When I googled it I found this link at wikipedia "http://www.cnac.ca/data/ac647.htm". It lists a similar number "(647) 580-8303 " as a Telus Communications number. The companies seem to use blocks of similar numbers, probably so they are easier to remember.I then checked in a phone and pulled up some debug info that lists a SMSC (Short message service center) number, a fancy way of labelling the network element in the mobile telephone network which delivers SMS messages (wikipedia). It then lists that number beside it. Since the phone was with Telus Communications it stands to reason that it would use Telus's SMSC, thereby proving my theory.

According to my cell phone bill I was texting someone at (647) 580-0172-- I don't even know anyone from Toronto (where on the bill where it says this number is from).

According to some quick research, I think this is from your service provider. When I googled it I found this link at wikipedia "http://www.cnac.ca/data/ac647.htm". It lists a similar number "(647) 580-8303 " as a Telus Communications number. The companies seem to use blocks of similar numbers, probably so they are easier to remember.
I then checked in a phone and pulled up some debug info that lists a SMSC (Short message service center) number, a fancy way of labelling the network element in the mobile telephone network which delivers SMS messages (wikipedia). It then lists that number beside it.

Since the phone was with Telus Communications it stands to reason that it would use Telus's SMSC, thereby proving my theory.

This is an internal number that Telus uses in conjunction with SMS. It is listed in the setup of my Telus smart phone.