New2This,
Please try the following code as it should assist you:
--Should be OnClick instead of OnClientClick.
<asp:TextBox ID="PhoneNumber" runat="server" width="120px"></asp:TextBox>
<asp:ImageButton ID="ImageButtonAdd1" runat="server" ImageUrl="~/Images/bullet.png"
Height="16px" Width="23px" AlternateText="Add another Phone Number"
CausesValidation="False" OnClick="TelNum2_Click" />
<asp:TextBox ID="PhoneNumber2" runat="server" width="120px" Visible="false"></asp:TextBox>
--For code-behind, make sure first phone number text box has a value and if so, show the second.
protected void TelNum2_Click(object sender, ImageClickEventArgs e)
{
if (PhoneNumber.Text.Trim().Length > 0)
{
PhoneNumber2.Visible = true;
}
else
{
PhoneNumber2.Visible = false;
}
}