Unlock old IPhone 6 to use with different carrier

I have an old IPhone 6 that was my daughters a long ago. We want to give it to our granddaughter to use as her first phone. It is currently locked. I've been trying to get it unlocked through T-Mobile for almost a month. They have the eimi #. They've come up with different reasons why it's taking so long. 1. It's being worked on. 2. because it hasn't been used in years it takes longer to unlock. And the latest 3. They need proof the phone was paid off. It's been at least 7-8 years since the phone itself has been active! My daughter had 2 more IPhones since the 6 and has moved on to her own plan and kept her phone number. The history on my account doesn't go back that far. And I no longer have paperwork or emails that the phone was paid. We've been with T-Mobile for years and before that Metropcs. How the heck do I get this phone unlocked to be used with a different carrier for my granddaughter??! Any advice would be great!