Verizon s3 with a Att gophone sim card

[AC Question]

Hi,so I have a verizon galaxy 3s ,put a att gophone sim card in and changeed apn settings. At first I checked mms messaging with another android phone ,now i have realized that my messages are reaching others but i am not receiving texts or mms. oh and not able to send mms? Do you think I will have to unlock the phone?