Ice (as in frozen water)
is generally very safe in places like Phuket. Almost everyone serving beverages to tourists buy their ice from the ice-factories. You will see the huge bags of ice being delivered in the evenings to all restaurants.
These ice cubes have, as stated above, a hole in it. This is the trademark of factory ice ( = 97% safe). If there is no hole, don't drink anything.
But as with everything nothing is 100% guarantee. A "friend", a woman, had a soft, cool drink while having a massage. She had to go to hospital due to dehydration. Four days spent at the luxurious hotel, no sorry, Hostpital (looks the same) isn't the best kind of vacation.
But in general, is there hole in the Ice, you are probably as safe as home!