told major news outlets last week that  for the iPhone and iPod touch. Vonage is an Internet telephony firm that provides inbound and outbound calls over the public switched telephone network (PSTN). Vonage charges $18 to $35 per month for unlimited inbound calls.
The Vonage application will almost certainly be limited to use over Wi-Fi networks, a condition that Apple and AT&T have imposed on VoIP (voice over IP) calling. The FCC is looking into these sorts of limitations, however, and it's possible that Apple, AT&T, and other smartphone makers and carriers will be required to offer non-discriminatory access to VoIP programs using 2G and 3G networks in the United States.
Vonage hasn't announced precisely what features will be in the app, what it will cost (although it's likely to be part of a subscription package), or when it will be available.
With Vonage on an iPhone or iPod touch, you could have your home or office landline number also ring on your iPhone or iPod touch. And placing calls using the Vonage app while connected to a Wi-Fi network would also enable you to avoid using minutes from your cellular calling plan.
Although Skype nominally offers similar capabilities, you'd have to give out a Skype number that you're unlikely to use as your main line. Or, you could use Google Voice, if you can get an invitation, and give out that number, having Google ring your various cell lines, landlines, and Vonage lines.
Vonage and Skype have somewhat different businesses, despite both operating Internet telephony services. Vonage focuses primarily on public telephone calling, selling adapters that plug into regular phones or home phone systems on one end and broadband modems on the other with the goal of replacing traditional landlines. Vonage also offers Mac and PC software linked into a single account. The company has about 2.5 million subscribers.
Skype, by contrast, with 400 million registered users, charges no fee for its software or Skype-to-Skype calling among computers and smartphone software. Skype has a very small set of mostly out-of-date hardware options and doesn't seem interested in phone adapters, though a few independent companies offer Skype-to-phone adapter options.
Skype does have public network interchanges, and is on target to gross over $600 million this year from per-minute fees and calling subscriptions. Skype charges about $6 per month for unlimited inbound calls on a single phone number, voicemail, and unmetered North American outbound calls. (I use the term "unmetered" because neither service is truly unlimited; both impose fair and reasonable use terms.)
That said, Skype disclaims any interest in being a landline replacement, and doesn't offer 911 emergency services. Vonage ties directly into 911 services for people with phone adapters in a fixed location and offers a national 911 call center for those using its software phones.
Both companies let you purchase and connect multiple incoming phone numbers in different cities, states, or countries. And both firms have various unmetered subscription plans and cheap per-minute calling among countries, always far cheaper than incumbent telephone and cellular carriers.
Skype was just sold to a consortium of private investors; see "," 2009-09-01.