We were enthralled with Vonage as a reliable and cheap VoIP service for our home phone. But after several weeks of non-service because of an error in their OSS and incompetent support, we're making the switch to Skype.
Most of our phone bill goes to calls with Estonia. Now we have an Estonian SkypeIn number and voicemail for 30 euros per year. Our friends and family in Estonia can call us for free (Joi did a similar international arbitrage trick with Vonage). SkypeOut is used for calling a handful of people who aren't on the network.
Skype numbers are available for Denmark, Estonia, Finland, France, Hong Kong, Poland, Sweden, the UK and US (not in major area codes from a quick glance). When other countries with ethnic diasporas are supported, I'd expect a significant switch because of the allure this arbitrage. Then imagine if they supported multiple SkypeIn numbers, not just for arbitrage, but as a decentralized PBX.
Skype is hardly perfect. We use it for Socialtext and it frequently hangs when people are trying to get together for a timed call. But for consumer use with mobile phones as a backup, phone companies are scared for a reason.
