The police may want to search your cellphone for certain evidence during a criminal investigation. They may suspect you of having broken the law, and they think your phone will provide evidence like location data, text records, call records or evidence about websites you visited or searches that you carried out.
As a general rule, you do not have to provide consent to a search of your phone, even if the police ask. In order to compel you to allow the search, they would need to get a search warrant. Otherwise, your phone can simply remain locked, and they cannot force you to open it.
However, the third-party doctrine can sometimes change where they can gather the evidence that they need.
Who else had access?
In the process of using your phone, you may have voluntarily shared information with another entity. If so, you have reduced your expectation of privacy, and the police can sometimes get that information from the third party, rather than your phone.
For instance, perhaps the police want to see your location data to establish where you were at the time that the crime took place. They do not have a search warrant, and you refuse to unlock your phone.
However, you had used Uber on the day in question, so you voluntarily provided your location data to Uber through the app. It may be possible for the police to subpoena Uber and get access to this data. In the same way, they can sometimes get records of direct messages from companies like Facebook, or they may be able to get your search history from your internet service provider.
It is important for the police to follow all proper procedures when gathering evidence. If you are facing serious charges, especially if you believe the police may have illegally obtained that evidence, it is important for you to know what criminal defense options you have.