I just rattle a bucket and they come galloping over. Bella knows her name and is lead sheep, so 'Bella, Bella, Bella' will get their attention if I don't have a bucket. They occasionally get a bit of Beyonce - 'All the single ladies' - sung at them, but they're not generally impressed with that...
Next door uses 'Come on', down the road uses 'Pet, pet, pet'.