If you used to have an iPhone, you may still be set (on their system) to be receiving iMessages, which are not SMS messages, which is what everyone but Apple uses. Have your iPhone friends check the SMS box at least once when sending you a message (which should tell their phones to always send your number an SMS). Also if you were in an iPhone, ask at an Apple store if they can help get you removed from iMessage (you can only do it with an Apple product).