Why won't my carrier change?

Hi-

I currently have an iphone 4s. I have updated my phone to the newest version possible. I have jail broken my phone years ago. I am currently using t-moblie as my main carrier. On the top of the screen it currently says that i am under Verizon, which i am not. Every other function on my phone works like normal. Since the update the I not contacted people but was told that they were trying to contact me. I have went into the settings and have not found anything to switch the carrier. I have also tried starting my phone numeral times and have failed. I kept on taking my sim card out and replacing it but nothing happened.

Please answer my problem. Thanks in advance.