Change "reply with message" text I know how to change text for RELY WITH MESSAGE option, however 
"Can't talk right now" part can't be changed in setting. 

Is there an app or a modification that can change "Can't talk right now" part? 

To change text for RELY WITH MESSAGE OPTION go to 


Settings->Phone->Reply with Message
If your device is jailbroken, you can use this solution to change the default reply message prefix:


  iFile will let you edit the .plist strings. How ever you do it, the .plist file is in /System/Library/PrivateFrameworks/TelephonyUtilities/English.Iproj folder (if you use English. What I don't know is if this changes anything in the sent text