iMessege Problems

After swtiching phones from iPhone to a different OS my firends are still having problems texting me. I unregistered my device on apple support, and deleted it form itunes. I don tknow what else to do. Im getting texts but my brother says it tried to send imessege fist everytime and then sends normal so its annoying for him. My other friends text dont even come through and just still try to send imessege. HELP any other suggestion? im sure i dont know of some texts im missing.