javascript question --auto tabbing using ascx control

Hello Expert,

This is bascially a javascript question:

Here is my ASCX user control code in asp.net.

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<!-- VALADATION JAVASCRIPT IN HELPERFUNCTIONS.JS FILE-->
<script language="JavaScript">
      function KeyUp1(what) {  
        if (what.value.length >= 3)          
            document.all("txtContactPhone_idExchange").focus();    
                  }
                  
            function KeyUp2(what) {  
        if (what.value.length >= 3)          
            document.all("txtContactPhone_idLastFour").focus();    
                  }
                                    
</script>
<asp:textbox id="idAreaCode" title="Enter Phone Area Code Number" runat="server" maxlength="3"
      width="40" onKeyUp="javascript: KeyUp1(this);"></asp:textbox>
<b>-</b>
<asp:textbox id="idExchange" title="Enter Phone Exchange Number" runat="server" maxlength="3"
      width="40" onKeyUp="javascript: KeyUp2(this);"></asp:textbox>
<b>-</b>
<asp:textbox id="idLastFour" title="Enter Phone Last Four Number" Runat="server" MaxLength="4"
      width="45"></asp:textbox>
<asp:Label ID="lblExt" Runat="server" text=" Ext:"></asp:Label><asp:textbox id="idExt" title="Enter Phone Extension Number" Runat="server" MaxLength="5" columns="5"
      width="45"></asp:textbox>

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I am doing auto tabbing for phone number user control.

i want the javascript to be more generic.

for eg:  document.all("txtContactPhone_idExchange").focus();    

txtcontactphone is the usercontrol name on my asp page.

idexchange is the textbox name on the ascx -usercontrol page

If  i want to use same user control  mutiple times on the same aspx page, i have to provide different names to the control for use.

so basically i cannot hard the usercontrol name here in the ascx control.

can any one help me out.

thanks