Usually this error message is caused by a single quote present in your input textboxes or by the use of a reserved keyword. Both problems are present in your query. The TABLE word is a reserved keyword for SQL Server and thus you should encapsulate it with square brackets, while for the possible presence of a single quote in the input text the correct approach is to use Parameterized Query like this
SqlCommand cmd = new SqlCommand("insert into [Table] values(@fnam, @lnam, @pNum)", con);
cmd.Parameters.AddWithValue("@fnam", txtfName.Text );
cmd.Parameters.AddWithValue("@lnam", txtlName.Text );
cmd.Parameters.AddWithValue("@pNum", txtpNumber.Text);
cmd.ExecuteNonQuery();
With this approach you shift the work to parse your input text to the framework code and you avoid problems with parsing text and Sql Injection
Also, I suggest to NOT USE a global variable to keep the SqlConnection reference. It is an expensive resource and, if you forget to close and dispose it, you could have a significant impact on the performance and the stability of your application.
For this kind of situations the using statement is all you really need
using(SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings
["ConnectionString"].ConnectionString));
{
con.Open();
SqlCommand cmd = new SqlCommand("insert into [Table] values(@fnam, @lnam, @pNum)", con);
cmd.Parameters.AddWithValue("@fnam", txtfName.Text );
cmd.Parameters.AddWithValue("@lnam", txtlName.Text );
cmd.Parameters.AddWithValue("@pNum", txtpNumber.Text);
cmd.ExecuteNonQuery();
}
Of course remove the global variable and the open in the Page_Load