I can imagine it was much more difficult for police detectives to convict criminals prior to the invention of cell phones.  They actually had to get search warrants to tap suspected criminal's home phones, or even tougher to tap public phone booths. That sure has changed today, as most officers know that all of our lives are contained in our cell phones.  Our phones keep track of where we go through GPS, all of our conversations are documented in the form of phone calls, text messages, emails, or chat apps. 

It is not uncommon today in a drug case today, for instance, to see a clear order for drugs, a negotiation for the price, and then discussion of who, where, and when the drugs will be delivered.  All of this is documented on the user's cell phones, and it is all accessible by law enforcement with the right tools.

In many of the cases I handle, I see law enforcement seizing cell phones.  They are allowed to seize the cell phones without a warrant if they have probable cause to believe that a crime has occurred, and that the cell phone will contain evidence of that crime. They still have to seek a search warrant to examine the phone, something a judge has to authorize, but they can seize the phone while they seek the warrant.  

Even if they have the phone, and have a search warrant, they still have to get into the phone.  On some phones this is incredibly easy, such as when the phone is unlocked and does not require a pass code of some kind.  If the phone is locked, and it can be unlocked by a fingerprint or facial ID, a judge could order a defendant to unlock the phone with a fingerprint.  I have observed that Android phones are incredibly easy for law enforcement to get into, and they simply plug their software into the phone and unlock it most of the time. 

If you want a cell phone that is difficult for the government to search, buy an iPhone.  Apple values their customer's security, and has created cell phones that are very difficult to unlock by anyone. Apple themselves cannot unlock the individual phones, and has purposely not programmed any "back doors" into their operating system, in order to ensure that their customers data is private and secure.  There are some companies that claim that they can access iPhones, but the cost to secure these services usually means that law enforcement agencies are not using them in the average case. 

If you buy an iPhone, you still should set it to the highest level of security.  This means setting a numerical pass code (not a fingerprint or facial ID), setting the phone to wipe itself if the wrong code is entered too many times, and not storing information on iCloud, that you don't want law enforcement to access.

The iCloud can be accessed by law enforcement with a search warrant or various other legal means. Apple publishes statistics about how often they turn over information to law enforcement.  In 2017, in the United States, they had the following requests:  

8,929 device requests received; 

24,126 devices specified in the requests;

7,113 device requests where data provided;

and in 80 % of device requests data was provided.  (Click Here For Documents)

It is also important to update your phone regularly, as Apple regularly makes changes to their operating system, in order to shut down security holes.  Ultimately if someone wants to get your data, they may be able to, but you should do what you can to protect your data and never consent to a search by law enforcement without a warrant, and NEVER give out your pass code without speaking to your lawyer first.