Voice over IP pioneer Vonage, already on Wall Street’s slippery stock slope, now faces an unexpected patent challenge from Verizon.

The telecom giant filed a patent infringement suit June 12, claiming Vonage is violating seven of its patents that cover the way phone calls are transferred from an IP network to the PSTN, billing and fraud detection, call services including call forwarding and voice mail, and the use of Wi-Fi handsets in a VoIP network.

Vonage officials said in a statement yesterday that the company is denying the charges and will “vigorously” defend itself in court.

The lawsuit is just the latest problem for Vonage, whose stock slipped further to $8.48 Monday. Vonage shares have already lost half of their value since the IPO was released at $17 in May.

Vonage also faces a class-action lawsuit over claims it violated securities laws when it pre-sold 13.5 percent of its stock offering to customers of its service.

Verizon stated in its lawsuit filing that Vonage is “aggressively” marketing its VoIP service to Verizon customers but didn’t seek an injunction against the service provider in advance of the lawsuit resolution.