Caller ID apps are extremely fickle in that they often only work for certain carriers and in general, never work when you need them to. Privacy Star has a decent caller ID system but they tend to fall short when it comes to Verizon numbers. Adaffix has a free Caller ID app in App World that is worth a try although they seem to be very European centric. The app also instantly provides alternative merchants for a business that you have been unable to get through to. A 3G connection is required for the Caller ID feature and suppressed numbers cannot be identified.