On what basis? spdustin cites United States v. Hubbell elsewhere in this thread, but I don't think that case establishes that a defendant cannot be compelled to do anything that would be "part of a chain of events that results in the production of incriminating evidence that the prosecution isn't otherwise independently aware of."

Hubbell suggests that "being forced to surrender the key to a strongbox" does not violate the Fifth Amendment. Under spdustin's generalization, the strongbox hypothetical would violate the Fifth Amendment.

Hubbell is based on the idea that providing documents responsive to a subpoena is itself a testimonial act, because it is "necessary for [the Defendant] to make extensive use of 'the contents of his own mind' in identifying the hundreds of documents responsive to the requests on the subpoena." I don't see how the purely physical act of unlocking the phone is similar.