Problem: Modifying existing php code to add telephone validation to form. Have the correct preg-match but not working. Any text entered is accepted

I was using some pre-existing open source code and wanted to add a telephone validation to a form. The original code did not have a telephone validation. I tried to mirror the email php validation but modifry the preg-match for telephone digits, spaces, brackets and hyphens. The form still works fine except that it is not validating the telephone section. From testing the form, I can confirm that the telephone addition to the form is not being recognized (I know this because I tried copying the email validation into the telephone code and it did not recognize that either). My code is below. I am certainly a beginner with php and really only use php for handling the emailing of forms. Any guidance is appreciated. I am hoping to achieve the telephone validating correctly when the form is submitted.

Code:

<?php

// Set email variables
$email_to = '[email protected]';
$email_subject = 'New email from form';

// Set required fields
$required_fields = array('firstName','lastName','telephone','email','comment');

// set error messages
$error_messages = array(
    'firstName' => 'Please enter your first name to proceed.',
    'lastName' => 'Please enter your last name to proceed.',
    'email' => 'Please enter a valid email address to proceed.',
    'telephone' => 'Please enter a valid telephone number to proceed.',
    'comment' => 'Please enter your Message to continue.'
);

// Set form status
$form_complete = FALSE;

// configure validation array
$validation = array();

// check form submittal
if(!empty($_POST)) {
    // Sanitise POST array
    foreach($_POST as $key => $value) $_POST[$key] = remove_email_injection(trim($value));
    
    // Loop into required fields and make sure they match our needs
    foreach($required_fields as $field) {       
        // the field has been submitted?
        if(!array_key_exists($field, $_POST)) array_push($validation, $field);
        
        // check there is information in the field?
        if($_POST[$field] == '') array_push($validation, $field);
        
        // validate the email address supplied
        if($field == 'email') if(!validate_email_address($_POST[$field])) array_push($validation, $field);
        
        // validate the telephone supplied
        if($field == 'telephone') if(!validate_telephone($_POST[$field])) array_push($validation, $field);
    }
    
    // basic validation result
    if(count($validation) == 0) {
        // Prepare our content string
        $email_content = 'New Website Comment: ' . "\n\n";
        
        // simple email content
        foreach($_POST as $key => $value) {
            if($key != 'submit') $email_content .= $key . ': ' . $value . "\n";
        }
        
        // simple telephone content
        foreach($_POST as $key => $value) {
            if($key != 'submit') $telephone_content .= $key . ':' . $value . "/n";
        }
                
        // if validation passed ok then send the email
        mail($email_to, $email_subject, $email_content);
        
        // Update form switch
        $form_complete = TRUE;
    }
}

function validate_email_address($email = FALSE) {
    return (preg_match('/^[^@\s]+@([-a-z0-9]+\.)+[a-z]{2,}$/i', $email))? TRUE : FALSE;
}

function remove_email_injection($field = FALSE) {
   return (str_ireplace(array("\r", "\n", "%0a", "%0d", "Content-Type:", "bcc:","to:","cc:"), '', $field));
}

function validate_telephone($telephone = FALSE) {
    return (preg_match('/^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/', $telephone))? TRUE : FALSE;
}

?>

I have tried copying the code from the email validation and changing the variables to $telephone. I then took the preg_match validaiton code and replaced it with a telephone validation code ('/^(?([0-9]{3}))?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/'). When the form is submitted, any text you add to the telephone form box is accepted and emailed.

The expecation I had was that the form would return a message in the form stating "Please enter a valid telephone number to proceed."