Bills I've never received sent to the collection service.

I just switched to T-Mobile on January 2014.  I just applied to the new apartment and when they did the credit report check,

I found out that T-Mobile sent something (I believe bills) to the collection service without noticing / sending me on the month I switched to T-Mobile.  I make a payment on time and I had a great credit score, but due to what they did, my credit score dropped 50points

(probably I didn't know about the collection service for last 5 months!) and my apartment application still could be approved,

but will be asked to pay much higher deposit.

I really want T-Mobile to fix this issue and give my credit score back!