Why Is It "Roger" At End Of Radio Convo?

Question:
Just changed taxi firm and we are on two-way radio.  Why do we have to say 'Roger' on the radio and not something else, for example Trevor?  What’s symbolic about it? 
Paul, Chesham

Answer:
Name: Andy, Slough
Qualification: Ex Military
Answer:  'Roger' used to represent R in phonetic alphabet.  Before Romeo and it was the first letter of Received.