Networks carrying both voice and data VLANs are vulnerable. Hackers publicized how they used freely available network scanning software to compromise a hotel network, gaining access to the hotel's internal corporate network.

Registration hacking is a way to listen in on others' calls. It requires the hacker to gain access to the target's registration authentication credentials. The hacker sends a REGISTER request to the Registration Server. The command directs all calls intended for the targeted recipient to the hacker. Since it is possible to have multiple registrations simultaneously, the call will go to the intended recipient and to the hacker. Use of a secure authentication method protects against this type of threat

Vishing is the voice equivalent of phishing. Instead of email with an imbedded link, the victim receives a phone call from a bank or credit card company. The victim is requested to call a specified number. The recorded message at that number requests account information. Individuals who would not be deceived by phishing have fallen victim to vishing.

Spam over Internet telephony (SPIT) can be even more aggravating than email spam. Infected zombies can be used, just as they are used to generate spam, to increase the volume and camouflage the message source. Both vishing and SPIT could be generated via the PSTN using automated phone equipment, but it would require dialing individual phone calls. Vishing and SPIT messages can be generated by the thousands. Receiving dozens of calls each day advertising drugs or pornography will drive users to switch back to traditional phone service.