In this research we have studied several human factors problems that are connected to the deployment of speaker verification technology in telecommunication services. We investigate the perception of the safety of a calling card service when it is protected by speaker verification on the 14 digit card number, and compare it to the perceived safety of speaker verification and PIN. Moreover, we compare a voice based interface to the service with a DTMF based interface. The results are crucial for guiding the introduction and deployment of speaker verification technology in actual applications.