Cell phones are small objects that can hold a lot of information and secrets. When the police want to look through it, in most cases, they will need a warrant. Instances where a warrant will not be required are:
These are general exceptions to the warrant issue, and truthfully, it does sound like many exceptions. However, each exception goes into much more detail, and in actuality, a more specific instance within one of these general situations may suddenly require a warrant. If you are confused, just keep doing research and talk to a lawyer for the specifics.