The biggest con depends on how reliable your internet service is. Phone service is generally more reliable than internet--but VOIp is totally dependent on your internet and electric service.

There can also be installation issues, depending on where your internet service comes into the house.
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The big con is that you're dependent upon an internet connection AND electricity. If the power goes out and you have an old phone lying around, the phone lines will usually still work. Also, there are sporadically noise artifacts on VoIP, like a slight echo effect.
Source(s):
Experience is my source.

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Vonage only works if your ISP connection is up, so if you get your data service over cable and your cable goes down, you lose phone service as well.

Vonage will not work when the power goes out since it requires a ATA (analog telephone adapter) device to act as the telephone to ethernet adapter. If the ATA loses power, then no phone. You can however connect your ATA to a UPS (uninteruptable power supply), just make sure you also connect your cable modem or DSL modem and any ethernet switch in between to the UPS.

You must register your address with Vonage in order for 911 calls to be routed correctly. If you take your ATA with you on a trip you should not use it to dial 911. Of course, you can't take a land line from the phone company with you on a trip.

I have used vonage for years including 2 years with a US number that I used from Europe. It is a great service and I have never had a major problem with it.
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Only one: your connection to the Net. If your Internet connection has bad quality or your bandwidth is limited then you'll have bad phone service.
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