Signed up for free trial, now I'm getting a bill and notice from debt collector.

I signed up for home internet and phone SIM trial because of a rep on T-Mobile website. I cancelled my plan and account with them before the trial ended. I never even activated any of the services. I was assured by the customer reps that the bill will go away when I asked them about it while cancelling. Now I have a bill from T-Mobile. and a notice from their debt collector. Anyone else know what to do?