IMessage doesn't work with all my iPhone contacts?

I'm on a pay as you go plan with my iPhone 4S and it's currently out of time but I'm able to text one of my friends who has a 3GS (cause iMessage is free between iPhone users?) but it doesn't work with texting my brother who has a 4? I have they're phone numbers under "iPhone" can anyone help me out?

Imessages sends messages via e-mail address that the other person uses as their apple ID, if you use imessage with their phone number it'll send as a text message.

IMessage is a new concept for iPhone 4 users and up. IPhone 3 users don't have iMessage, so you won't be able to contact them that way.

Make sure you're brother is either connected to Wi-Fi or a cellular data network. Also check that he has iMessage enabled under Settings>Messages>iMessage

Both devices must have iMessage turned on. You can iMessage phone numbers or Apple IDs associated with iOS or OS X devices.

My mom and I both have iphone 5's and we used to be able to iMessage each other. One day it just suddenly stopped working. I never understood why that happened. Any ideas or explanations? I'd be happy to know!

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