I'll leave the final ruling to --
-- actual judges. It seems tricky.
In USA, police can enter and search your house if they have a warrant. They can open and read any mail they find. In the case of a safe, they can ask you to open it, and if you refuse then they can have a locksmith go to work on it. Or have it cut open.
So if the cell phone contains the equivalent of personal correspondence -- mail -- then a warrant allows them to open it. Or if it is equivalent to a safe, then they can indeed ask you to open it, or else have it cut open. Open it and you may get points for cooperating with the investigation; make them break it open and you get no points and you still get pwned.
However, the contents of the phone are not like the contents of your memory. They can't legally search your memory, ie, by waterboarding you until you "remember" anything they tell you to remember. And the passphrase for the phone is in your memory. Or, if a bishop, it's tattooed on the back of your neck.
So I don't find the situation clear enough to pretend I know the answer.
Oh, and Yet Another AC: speaking of mobster-era seizure rules, most US states right now allow the police to seize cash and property if they merely suspect (or pretend to suspect) that said cash and goods were obtained illegally. They don't have to give it back if the owner is never arrested or charged with a crime, either. Your unexplained cash wad is all profit for the boys in blue.