Messaging problem

So yesterday I was able to text my friend who had an android. But this morning when I woke up and tried to text them this message keep showing up, “ Free Msg: Unable to send message - Message Blocking is active.” And it is really frustrating since i don’t know why this is happening. It had happened few times before and then it gets fixed. But I’m not sure why this problem is keep occurring. So if you can please help with this problem it would be very much helpful. I have the iPhone 6s Plus, 32gb. Thankyoy