In an effort to create a benchmark for antenna designs for mobile applications, the total and radiation efficiencies of a modern high-end smartphone’s antennas are measured. To this end, an adapter board is designed that facilitates the connection to a calibrated measurement system. Its shape is chosen to closely resemble the original phone’s dimensions while allowing connection of the antennas via spring fingers to a coaxial measurement system. The characterization of this mock-up is performed in a reverberation chamber, which offers the advantage of an on-average isotropic environment, making it ideal for antenna reflection coefficient and efficiency measurements. The reflection coefficients, total efficiencies, and radiation efficiencies of the mock-up are then measured from 0.75 to 6 GHz. The total antenna efficiencies, not accounting for a possible improvement due to adaptive matching networks, are found to be below 15% up to 4 GHz and up to 25% below 6 GHz. Meanwhile, the radiation efficiencies are up to 15% below 2.5 GHz and up to 40% below 6 GHz. Such antenna efficiency measurements are the first and serve well as a benchmark for future designs and concepts.