@Julie *************
it's very possible that Phuket cannot do anything to help you since they didn't give you the entry stamp.
The only extension you can get if they can't fix the stamp is the 30 day tourist extension which is 1900 baht.
If the immigration office in Phuket can't fix it, the only thing you can do is leave the country and re-enter and make sure they stamp you in correctly with your visa.
If you get the tourist extension you lose the ability for anyone to fix the entrance stamp and then leaving and re-entering will be your only option.