Debt collector calling and texting but no mail.

My girlfriend cancelled her T-Mobile last year. Prior to cancelling we received several different accounts of whether or not my girlfriend had to pay anything to TMobile (final bill, etc) in order to do so. The last version of what we heard (after being told she wouldn’t have to pay at all) was that a final bill would be sent in the mail.

Fast forward a month and we never received anything in the mail. She reached out to T-Mobile over the phone to get clarification. They basically said they couldn’t help her because at that point her account was gone and she no longer had access.

Fast forward again to today. For about two-three months or so she’s been getting phone calls and text messages randomly from a debt collector (verified that there’s a debt collector with this name with a Google search) saying she owes about $200 to TMobile. They never leave a voice mail and she has never talked to anyone or responded to the texts. Over the last few weeks the texts oscillated between “paying off the debt is urgent and something needs to be done immediately or there will be further consequences” to “your debt is delinquent” to “hey you have this debt and if you pay it now get 70% off”.

We still have yet to receive anything physically in the mail from T-Mobile or the debt collector. And nothing else has come of it. Her credit score is still as high as it was and everything is fine there.

Not sure if we keep ignoring or what. I don’t want her to get in trouble but I also still don’t know if this debt collector is even 100% legitimate given they’ve never sent anything official. What’s the best play here?