The general rule is that officers need a warrant before they can tap a cell phone.  If officers illegally search and seize your information, it could later be subject to suppression.  That being said, it does not mean that officers sometimes bend the rules.  Moreover, if the officers are smart they may use the information that they illegally obtain to give them a lead to catch you in the act of doing something illegal, thereby bypassing the need to get a warrant.  My advice, if you think that officers may be monitoring your activity stay out of trouble.  You are much better off preventing getting arrested rather than getting arrested and trying to have your attorney argue that evidence should be suppressed.