<a href="#" class="replace"></a>
<input type="hidden">

let link = document.referrer,
    replace = document.querySelectorAll('.replace'),
    replace_input = document.querySelectorAll('input');

// 100

    if (link == 'a.domen.ru'){
        for(let i = 0; i <= replace.length; i++){
            replace[i].innerHTML = '+7812';
            replace[i].setAttribute('href','tel:+7812');
        }

        for(let i = 0; i <= replace_input.length; i++){
            if(replace_input[i].getAttribute('type') == 'hidden'){
                replace_input[i].value = link;
            }
        }
    }