Just the act of refusing makes the act of seizing your phone legal or not. If you legally give them your phone by your own will, they are able to use all evidence they find in the courts. If you deny to give them your phone, and they seize it anyways and access it you have a valid path to throw the evidence they discover out as an illegal search and seizure of your property. I’m not a lawyer but that is the general thought process on denying them access to your property.
This is good to know, but adds an additional step to simply requiring a passcode to unlock on screen lock.