[Strongman]

<select id="s1" name="nomer">
    <option value="tel:123456" selected="">123456</option>
    <option value="tel:234567">234567</option>
    <option value="tel:345678">345678</option>
</select>
<a id="a1" href="tel:234567">Позвонить</a>

<script>
var smy = document.getElementById("s1");
var assilka = document.getElementById("a1");

smy.onchange = function OnChange()
{
assilka.href = "tel:12345";
}
</script>