My take on this dog is that it is probably a Biewer due to the silky coat I see. A Biewer that is mixed with a Shih Tzu has a very fluffy coat. Also there is a Biewer breeder in Arizona about 2 hours away from Phoenix (sad the dog was not micro-chipped). To my knowledge there are no Parti breeders in Arizona. Biewers come with many different looks and if you have a breeder that doesn't really know what they are doing this face/muzzle could happen. I also believe I see a little tan in the face and whomever owned this dog let its ears grow and flop down. The Parti's and the Biewers original beginnings came from the same kennel out of England and both dogs are the same. The US breeders took the breed one way and the Germans took it another way. A dock tail is also not an indicator as a Biewer breeder in California was docking the tails of his dogs.
This little guy has already been fixed and is in a rescue situation so he will find a great little home and not produce any more Biewers like him.