When replying to ATC, callsign first or last?
ICAO Annex 10 - Aeronautical Telecommunications Vol 2


When replying, you end the message with the call sign.



  5.2.1.9.2.2 PANS.— An aircraft station should
  acknowledge receipt of important air traffic control messages
  or parts thereof by reading them back and terminating the
  readback by its radio call sign.
  
  Note 1.— Air traffic control clearances, instructions and
  information requiring readback are specified in PANS-ATM
  (Doc 4444).


Exception to rule:


  5.2.4.4.1 PANS.— When an aeronautical station initiates
  a call by SELCAL, the aircraft replies with its radio call sign,
  followed by the phrase “GO AHEAD”.


However, local procedures should be followed if they are different. See the post, 'Is it bad practice to say my callsign first in a transmission to ATC?' for an answer about UK and Germany. Basically when replying you finish with the c/s, when requesting you begin with the c/s.