Best app in this case is 'Note Listen for Deaf' .
iTunes:https://itunes.apple.com/us/app/note-listen-for-deaf/id660131784?mt=8
Small Video :
Also you can use 2 devices with 'Deaf Hearing Chat' app system (iOS or Android):
Positive review from US user
iPhone app 'Note, Listen for Deaf'
by scenicbrook
I am a deaf person and this application is great!! I can hand my phone to anyone, they speak into the phone and it converts the speach to text. I read lips and find it very helpful in restaurants where lip reading is...
A handy application with speech to text translation for deaf people, allowing a deaf person to write a note for a hearing person. This note will be displayed in large type and will be clearly visible on the device screen. This application allows a deaf person to talk with a hearing person...
My deaf son use android phone and tablet. Phone and tablet connected via bluetooth. He gives tablet to doctor and then they can exchange messages and freely communicate.