Cell phone searches without a warrant are unreasonble searches and illegal. Anything found in that search may not be used against the person arrested. However, if there is sufficient information to get a warrant to search the contents of the phone, then it may be searched. There are also possible exigent circumstances that may justify the cell phone searches without a warrant, like officer safety or destruction of evidence.
Cell phone searches are legal by a police officer if he has a search warrant. The U.S. Supreme Court decided in the case of Riley v. California just this June, 2014, that in most cases a search warrant is required. There may be some exigent circumstances which could possibly excuse the warrant requirement, but that was not an issue nor really addressed in the Riley case.