This work presents an implementation of spectrum handoff and selection in order to experiment a Voice over IP (VoIP) application over a cognitive radio testbed. VoIP connections use both WiFi bands and GSM bands to select available channels dynamically. We use the USRP/Gnu radio testbed to implement spectrum management functions and to run the experiments. We also experiment VoIP and data transfers together by implementing a satisfaction-based inter-service cohabitation strategy. Through subjective observations and objective measurements, we found that the quality of established phone calls can be maintained. Besides, VoIP can benefit from the flexibility of data transfers to perform handoffs more adequately. In the same time, data transfer is able to sustain its required average rate with minimum degradations.