I cannot change the message

i cannot change the message alert tone on my samsung u600 phone which is strange as i only bought it in january of this year and i could change the message alert tone until the phone went faulty and i had to exchange it which it has been exchanged twice now as it could not be fixed when sent back to the manufactuar and now i cant change the message alert tone it wont let me change the setting does this mean that im stuck with the hideous t mobile ring