As voice over IP (VoIP) services are becoming more widespread, UDP-restricted stateless firewalls (URSFs) have emerged as one of the main hurdles for deploying the service. By overcoming this obstacle, the new VoIP service Skype has gained rapid popularity. To traverse URSFs, Skype uses an abnormal transport method, namely voice over TCP (VoTCP). This paper investigates the effects of VoTCP on communication quality, and examines its operational network conditions. The experimental results show that the adoption of TCP for voice traffic causes a remarkable deterioration in the quality of service; however, VoTCP is applicable in a relatively clean network, such as the Skype's VoTCP section between a client and the client's super node. More specifically, if the jitter buffer size is adequately tuned, VoTCP can operate at a random packet loss probability of less than 1.6%.