[Skipp]

можно так

<input onkeydown="this.value=this.value.replace(/\D/ig,'')"/>

а лучше типо того

<input class="txt-phone" id="phone" type="text" maxlength="12" />
<script type="text/javascript"> 
	document.getElementById('phone').onkeydown = function(e){
		var e = e || window.event;
		var key = e.which || e.button;
		if((key<48 || key>57)&&(key<96 || key>105)&&key!=8)
		 return false;
	}
</script>