Billing issue

I switched from Cricket to T-Mobile on December 27 at the Walmart phone center. The Walmart sales representative falsely assured us that we would receive a free phone, but while entering our information into the system, she informed us that we would actually have to pay for the phones. By that point, we had already transferred two lines to T-Mobile, but upon realizing the misinformation, we immediately stopped and did not transfer the other two numbers.

On the same day, we returned the phone and SIM card and canceled the service with T-Mobile. The T-Mobile representative assured us that there would be no charges. However, due to waiting for Cricket to mail our SIM card, our number was not ported back to Cricket until December 30.

Today, I received a bill from T-Mobile for $110, despite only using the phone on December 27 to contact T-Mobile customer service. I had the phone and SIM card for only six hours. When I reached out to T-Mobile via online chat, they claimed I kept the phone numbers for three days.(I returned the phone and the simcard at walmart at the same day)

Who should I contact to resolve this billing issue?