Study Abroad Phone Bill collections

I studied abroad in Australia in the Spring of 2016 and had a phone serviced through Vodafone. I distinctly and specifically remember canceling my phone account before leaving the country, but I did it over the phone (and I'm not the guy to write the conversation reference number down) so there is no evidence of my phone call. I was under the impression that I'd done everything necessary to end the account. I can't even sign in to an online account anymore.

A few months after getting home to the US, I received a phone bill of $164, which was way more than my usual bill of $45. I have attempted to combat this charge many times through Vodafone, to no avail. A few months ago I got a notice that my outstanding balance was sent to a collections agency, so I disputed the claim through them again. Today I received a settlement offer for $84 from the collections agency.

Do I pay to settle the claim? Is paying this worse for my credit? Do I do nothing forever? Help Reddit!

Cheers