Is the phone under contract? If so your options may be limited. If you own it outright then I would go to a T-Mobile store and have the representative put in a T-Mobile SIM card and see what happens.
The iPhone 5C's do come factory unlocked from Verizon. In fact any phone that is global capable (CDMA and GSM) has come unlocked from Verizon for about 2 years. Just go to the store and set everything up. They can use one of their sims just to make sure.