IPhone data ran out, how to switch to text message?

I hope I don't confuse anybody with this question, my data has ran out and I have turned off my cellular completely so I can still receive and send texts and phone calls but the question I need answering is how do people text me when they have an iPhone as their message always sends as IMESSAGE. Is there a way for them to be notified that my iMessage technically is off as my data is off so their message will send as a text so I can read it. At the moment I have to wait until I get to Wi-Fi and then all the messages come through

Just send them a message on facebook