Add a permanent prefix to a textbox
===

What I am trying to achieve is to force a textbox to start with a prefix ( country telephone code ) and to make this permanent, meaning that the user cannot bypass this. For example, the Phone textbox should always start with "+45" and after that the user can add the phone number. How to prevent it from deleting the code, by any means?

What I done so far, using jQuery:

    //attach event on phone text boxes
    $(document).delegate(".phone", "keyup", function(event){
	
	var target = $(this);
	
	var value = target.val().trim();
		
	if (value.indexOf(CONSTANTS.DANISH_PHONE_CODE) == -1) {
		//country code not found
		
		//if the user starts deleting the country code 
		if (value.indexOf("+") == 0){
			value = "";
		}
		
		//if the user types something in front of the country code, put the country code at the end
		value = value.replace(CONSTANTS.DANISH_PHONE_CODE, "");
		
		//phone doesn't start with +45
		value = CONSTANTS.DANISH_PHONE_CODE + value;
		
		target.val(value); 
	}
	
	
    });

But the problem is that the user can delete the plus sign and the prefix is put automatically at the start so we will have +4545. Do you know an elegant way of achieving this? Thanks.