It's a 5th Amendment issue, or at least it should be. I realize I'm not Steve Bannon, but I haven't done anything to warrant a search of my phone. I would go to court & fight for my 5th Amendment rights against self-incrimination, though. They can subpoena the carrier for the information.
That's right, & the individual has the right to refuse to incriminate themselves under the 5th Amendment. It applies to electronics also. However the police can still confiscate the phone. Their IT people can work around it & gain access. Or the police can subpoena the phone records, but the contents of text messages are only stored by the carrier for 10 days. Call histories are stored longer.