Why I received friend iMessage & sender shown as his contact phone number?
Then this was not an iMessage. It was a text message. Ask your friend to look at their screen. They should see that the message they sent is in a green bubble.
For the receiver: All messages appear in gray.
For the sender: BLUE = iMessage
GREEN = Text (SMS) Message
The issue I had was with my partners phone. It was sending from her email which was splitting the message thread if she sent messages that came through as SMS rather than iMessage. So I had 2 split conversations running.
Its a simple fix on iPhone...
Goto.....Settings, Messages, Receive at, sign out of your apple ID.
If when you go in there and you are signed in you will see "You can be reached for messages at:"
If you are signed in it will use your email by default rather than your phone number. Signing out refers it back onto your number instead. Now all messages are from the one sender.
Hope this helps.