Dynamically enable or disable RequiredFieldValidator based on value of DropDownList

You can do this with JavaScript like this:

ValidatorEnable(RequiredFieldValidatorId, false);

Then have your drop down list use the onchange event (I’d suggest using jQuery)

$("#<%=dropDownList.ClientID %>").change(function(){
    var val = $(this).val();
    var skip = null;
    if (val == 1)
       skip = "workPhoneValidator";
    else if (val == 2)
       skip = "cellPhoneValidator";
    ....

    // by popular demand...
    var $skip = $("#" + skip)[0];

    if (skip != "workPhoneValidator") ValidatorEnable($skip, false);
    if (skip != "cellPhoneValidator") ValidatorEnable($skip, false);
    ....
});