Scam?

As of the past four days, I have been receiving numerous phone calls from 1-844-796-0648. I looked up the number and it seems to say on multiple websites that this is a scam claiming to be T-Mobile, so I have been ignoring the calls since.

As of yesterday, I received a letter in the mail saying that I have an outstanding balance. I checked my account and there is no outstanding balance. The letter looked suspicious to me considering it does not look like any other letter I have received in the past from T-Mobile (the paper is different, there is no color to the paper, etc.). I looked up the phone number provided in this letter as well which states it is the financial service department (1-888-310-8471). I found that this also seems to be a scam number as well. 

I guess my questions are: is anyone else dealing with this? is this actually a scam? if this is a scam, how did they receive my information (phone number and address)?