This work studies the feasibility of RF (radio frequency) harvesting of electromagnetic energy from intentionally generated RF signals in the 2.4 GHz ISM band as power supply for a battery-less (back-)scattering transponder. Our setup uses two regular, unmodified mobile telephones, one for transmitting RF signals and quasi-continuous Bluetooth (BLE) advertising signals, the other for receiving the scattered signals back-scattered from the transponder. The transponder comprises a harvesting circuit for accumulating energy from the incident RF signals to provide power to a control unit that modulates the incident BLE signal. This modulation applies frequency translation via 4 MHz subcarrier and frequency shift keying for communication such that the scattered signal is detectable by a BLE receiver of the receiving telephone. We compare different implementation technologies for bitstream generation and subcarrier modulation: hardware, microprocessor (MCU) as well as digital and analog designs. Experiments show that the combination of MCU and switchable analog subcarrier generator shows the most promising result with an active power consumption of less than 1.2 mW and an wireless range extension to 4 cm. Thus, the proposed design enables an improved communication between battery-less transponder and mobile telephone, which encourages further endeavors in this direction.