Vonage was for sure in the USA regarding the VoIP market a early pioneer. Unfortunately they under estimated intellectual property (patents) and after the big lawsuit with Verizon they never was able to set up a real good business model.
1. One big problem i believe they are having is - "total cost of VoIP infrastructure"
2. In the past, maybe not any more now they had a second problem with - "fluctuation of subscribers" and total cost to add new subscriber.