Como valida numero de telefono

[demoche]

Probé así :(

Código PHP:

<?php
require ("ClassMathGuard.php");
if($_POST)
{
$javascript_enabled = trim($_POST['browser_check']);
$department = trim($_POST['dept']);
$name = trim($_POST['name']);
$email = trim($_POST['email']);
$phno = trim($_POST['phno']);
$subject = trim($_POST['subject']);
$msg = trim($_POST['msg']);
$selfcopy = trim($_POST['selfcopy']);
 
 
//mail settings
$sales_address = "[email protected]";
$support_address = "[email protected]";
$billing_address = "[email protected]";
$headers = "From: ".$email;
$message = "Contact name: $name\nContact Email: $email\nContact Phone: $phno\n\nMessage: $msg";
 
if (MathGuard :: checkResult($_POST['mathguard_answer'], $_POST['mathguard_code'])) { 
 
    if ( $department == "sales") $to = $sales_address;
    elseif ( $department == "support") $to = $support_address;
    elseif ( $department == "billing") $to = $billing_address;
 
    if ( $name == "" )
    {
        $result = "Name field is required";
    }
    elseif (!preg_match("/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,4})$/", $email)) 
    {
        $result = "Enter a valid email address";
    }
    elseif(!preg_match("/^(([0-9]{2}-[0-9]{8})|([0-9]{3}-[0-9]{7}))$/", $celno)) //check for a pattern of 91-0123456789 ej: 221-4593367 
    { 
        $result = "Tipia bien el numero de celu!"; 
    }
        elseif(!preg_match("/^[0-9]{3}-[0-9]{8}$/", $celno)) //check for a pattern of 91-0123456789 ej: 221-4593367
    {
        $result = "Enter a valid celphone number";
    }
    elseif ( $subject == "" )
    {
        $result = "Subject is required";
    }
    elseif ( strlen($msg) < 10 )
    {
        $result = "Write more than 10 characters";
    }
    else
    {   
        
            mail($to, $subject, $message, $headers);
        if( $selfcopy == "yes" )
            mail($email, $subject, $message, $headers);
        $result = "Your mail has been sent succesfully!";
        
    }
}
else $result = "Bad Answer, Please try again!";
    
    if($javascript_enabled == "true") {
        echo $result;
        die();
    }
 
}
?>
 

Que hice mal?
Gracias de nuevo!