Try to unsign and sign in again.
For FaceTime and iMessage chosse to use your email address instead of your mobile number.
Go to Settings>>Message>>Send&Receive and set your email address.
If it is still not working, unsign your Apple ID and select your mobile number ("+" "country code" "mobile number").
Change your address for your currently location. If not works, change it for your native country.
Turn off Show Subject in Settings>>Message
Reset the Network Setting, which makes your phone to forget every networks and passwords that you connected your phone to.
to reset the Network Settings just go to Settings - General - Reset (at the bottom) - Reset Network Settings.
I remember when I reset the Network Settings, i turned off iMessages. I connected to my home Wi-Fi and surfed the web for a little while. Sent some regular SMS messages, and about 10min later, I turned on iMessages again and sent a text to a friend with an iPhone and it started working as iMessage! Baloons finally became blue.