public partial class Controls_ContactForm : System.Web.UI.UserControl
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
    {
      if (txtPhoneHome.Text != string.Empty || txtPhoneBusiness.Text != string.Empty)
      {
        args.IsValid = true;
      }
      else
      {
        args.IsValid = false;
      }
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="ContactForm.ascx.cs" Inherits="Controls_ContactForm" %>

<script runat="server">

</script>



<style type="text/css">
  .style1
  {
    width: 100%;
  }
  .style2
  {
    font-size: large;
    font-weight: bold;
  }
</style>
<script type="text/javascript">
  function ValidatePhoneNumbers(source, args) 
  {
    var txtPhoneHome = document.getElementById('<%= txtPhoneHome.ClientID %>');
    var txtPhoneBusiness = document.getElementById('<%=txtPhoneBusiness.ClientID %>');
    if (txtPhoneHome.value != ' ' || txtPhoneBusiness.value != ' ') 
    {
      args.IsValid = true;
    }
    else 
    {
      args.IsValid = false;
    }


  }
</script>
<table class="style1">
  <tr>
    <td colspan="3">
      <span class="style2">Get in touch with us</span><br />
      <br />
      Use the form below to get in touch with us. Enter your name, e-mail address and 
      your home or phone number to get in touch with us.</td>
  </tr>
  <tr>
    <td>
      Your name:</td>
    <td>
      <asp:TextBox ID="txtName" runat="server"></asp:TextBox>
    </td>
    <td>
      <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" 
        ErrorMessage="Please enter your name" ControlToValidate="txtName">*</asp:RequiredFieldValidator>
    </td>
  </tr>
  <tr>
    <td>
      Your e-mail address:</td>
    <td>
      <asp:TextBox ID="txtEmailAddress" runat="server"></asp:TextBox>
    </td>
    <td>
      <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" 
        ControlToValidate="txtEmailAddress" Display="Dynamic" 
        ErrorMessage="Please enter an e-mail address">*</asp:RequiredFieldValidator>
      <br />
      <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" 
        ControlToValidate="txtEmailAddress" Display="Dynamic" 
        ErrorMessage="Please enter a valid e-mail address" 
        ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*">*</asp:RegularExpressionValidator>
    </td>
  </tr>
  <tr>
    <td>
      Your e-mail address again:</td>
    <td>
      <asp:TextBox ID="txtEmailaddressConfirm" runat="server"></asp:TextBox>
    </td>
    <td>
      <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" 
        ControlToValidate="txtEmailaddressConfirm" Display="Dynamic" 
        ErrorMessage="Please confirm the email address">*</asp:RequiredFieldValidator>
      <br />
      <asp:CompareValidator ID="CompareValidator1" runat="server" 
        ControlToCompare="txtEmailAddress" ControlToValidate="txtEmailaddressConfirm" 
        Display="Dynamic" ErrorMessage="Please retype the e-mail address">*</asp:CompareValidator>
    </td>
  </tr>
  <tr>
    <td>
      Home phone number:</td>
    <td>
      <asp:TextBox ID="txtPhoneHome" runat="server"></asp:TextBox>
    </td>
    <td>
      <asp:CustomValidator ID="CustomValidator1" runat="server" 
        ClientValidationFunction="ValidatePhoneNumbers" Display="Dynamic" 
        ErrorMessage="Please enter your home or business phone number" 
        onservervalidate="CustomValidator1_ServerValidate">*</asp:CustomValidator>
    </td>
  </tr>
  <tr>
    <td>
      Business phone number:</td>
    <td>
      <asp:TextBox ID="txtPhoneBusiness" runat="server"></asp:TextBox>
    </td>
    <td>
      &nbsp;</td>
  </tr>
  <tr>
    <td>
      Comments:</td>
    <td>
      <asp:TextBox ID="txtComments" runat="server" Height="89px" 
        style="margin-bottom: 0px" TextMode="MultiLine" Width="470px"></asp:TextBox>
    </td>
    <td>
      <asp:RequiredFieldValidator ID="RequiredFieldValidator4" runat="server" 
        ControlToValidate="txtComments" Display="Dynamic" 
        ErrorMessage="Please enter comment">*</asp:RequiredFieldValidator>
    </td>
  </tr>
  <tr>
    <td>
      &nbsp;</td>
    <td>
      <asp:Button ID="btnSend" runat="server" Text="Send" />
    </td>
    <td>
      &nbsp;</td>
  </tr>
  <tr>
    <td colspan="3">
      <asp:ValidationSummary ID="ValidationSummary1" runat="server" 
        HeaderText="Please correct the following errors before you press the send button" 
        ShowMessageBox="True" ShowSummary="False" />
    </td>
  </tr>
</table>

if (txtPhoneHome.value != ' ' || txtPhoneBusiness.value != ' ')