I can not send any text messages from my 4.4.2 adroid phone- Can you help me?

[AC Question]

I have a 4.4.2 knock off android phone A7 Android- My phone suddenly quite sending messages- I have been with straight talk tech people 4 times- I have at & t as my carrier- They had me change my apn number and info and it still will not send messages-.. I have rebooted -taken the battery out-etc-. I'm very frustrated-!!!

Can you please help me?