If you're looking to buy the best dustproof and waterproof phone, you'll need to check out the IP rating. It tells you how much water and dust your phone or device can withstand. It's more precise than terms like 'water-resistant', but even devices with the same ratings can vary - check the individual specifications if you're keen to know.
'IP' stands for or 'ingress protection'. For devices, the first number tells you about the dust resistance. 5 means 'dust protected', so a bit of dust might get in but not enough to stop the device working. 6 means completely 'dust tight' - no dust will get in.
The second number is for liquids. Devices rated at 5 are protected against water spraying on them. Ratings of 7 mean the device is safe at between up to 15cm and up to 1 metre deep. A rating of 8 means the device is protected in more than 1 metre of liquid.
If there's an X, such as IPX8, it means the device isn't rated for that factor - so IPX8 would be tested for waterproofing but not dustproofing.
Some manufacturers provide more details, such as Sony. We'd recommend exploring manufacturers' websites for more information.