Gravity forms : verify phone number from Belgium

Some snippets to put in your functions.php and adapt if needed for language to trigger :

Mobile number

/* $phone_formats['fr'] = array(
        'label'       => 'Belgium mobile',
        'mask'        => '9999/999999',
        'regex'       => '/?([0-9]{4})\/?([ .-]?)([0-9]{3})\2([0-9]{4})/',
        'instruction' => __('Insert only digits from your mobile number', 'client_theme'),
    );

Phone number

$phone_formats['fr'] = array
      'label'       => 'Belgium phone',
      'mask'        => '999/999999',
      'regex'       => '/?([0-9]{3})\/?([ .-]?)([0-9]{3})\2([0-9]{4})/',
      'instruction' => __('nsert only digits from your phone number', 'client_theme'),
  ); 

Unified mobile and phone number

$phone_formats['fr'] = array(
      'label'       => 'Unified Belgium mobile and phone',
      'mask'        => '999999999?9',
      'regex'       => '/^0\d{9,10}$/', // phone at least 9 numbers
      'instruction' => __('Inserts only digits', 'client_theme'),
    );

Official Gravity forms documentation

More informations with maks and regex for phone numbers in Gravity forms documentation