Use the FieldGroup class
public function getCMSFields($fields) {
$fields = parent::getCMSFields($fields);
$fields->addFieldToTab('Root.Main',
FieldGroup::create(
new HeaderField('+36'),
new TextField('PhoneNumber','')
)->setTitle('Telephone number')
);
return $fields;
}
Personally I'd use two TextFields for cosmetic reasons, and enforce some validation on the TelephonePrefix.