if u r using html text and button inside aspx page then u should try like this ;
<input id="customerPhone" name="customerPhone" type="Text" />
<button OnServerClick=" Btn_Click" runat="server" id="Button1" />
// In .cs code write
protected void Btn_Click(object sender, EventArgs e)
{
string n = String.Format("{0}", Request.Form["customerPhone"]);
}