I don't have any advice, but it might be helpful to post in a geographical subreddit like r/Ontario if you haven't already. 

I would suggest doing a google search on your number and see what comes up

The ad is probably on backpage. Check all the backpage sites for your geographic area. Also google. 

My phone number somehow got online and now I'm being texted frequently by people asking for rates, availability, and saying how I'm attractive and whatnot. My number was posted online, as I have received texts from many different area codes, but all close to Toronto. How can I track down and remove this ad? I need advice asap, please and thanks!

My phone number has been used to sell sex and now I'm being spam texted