Making Contact Standard Fields Mandatory

To make contact standard fields mandatory, you must create a custom field of type Custom Field Type: Special – Browser Script and place it on the same entity that you'll be using it. For contacts, you'll want to put the custom field in via Global Settings > User & Contact Settings > Contact Custom Fields

function additionalCheckingOnSubmit(){
    var frm=document.frmuser;
    var element=frm.uaddress;
    if(element&&element.value.length==0){
        alert("Address is empty!");
        return false;
    }
    element=frm.phone;
    if(element&&element.value.length==0){
        alert("Phone is empty!");
        return false;
    }
    element=frm.ucity;
    if(element&&element.value.length==0){
        alert("City is empty!");
        return false;
    }
    element=frm.ucountry;
    if(element&&(element.value.length==0||element.value=="0")){
        alert("Country is empty!");
        return false;
    }
    element=frm.email;
    if(element&&element.value.length==0){
        alert("Email is empty!");
        return false;
    }
    return true;
}

function savefunc(){return additionalCheckingOnSubmit();}