Customers of SunRocket, the nation’s second largest provider of independent voice-over-Internet protocol (VoIP) services, were suddenly left without service last weekend as the company abruptly ceased operation without notifying its subscribers. Founded in 1994, Vienna, Virginia-based SunRocket reached the 200,000 subscriber mark last April and had been outranked only by VoIP industry pioneer Vonage Holdings. Although SunRocket offered one of the most attractive deals in the VoIP industry with unlimited calls throughout the U.S., Canada and Puerto Rico for $199 per year, experts say that SunRocket’s fate illustrates the difficulties faced by Vonage and other providers of stand-alone VoIP services, which are struggling to compete against cable operators that offer Internet telephony as part of a bundle that includes video and broadband. In the wake of SunRocket’s shut down, Vonage and other providers have offered their services to affected customers. Sources also indicate that Sherwood Partners, a California firm handling the wind down of SunRocket’s affairs, has reached agreements with IP telephony providers 8X8 Inc. and Unified Communications Corp. that name both companies as “preferred” service providers for former SunRocket customers.