Is that a V-Phone in your pocket?
While eminently portable (who can't carry around a tiny USB powered device?), the V-Phone probably won't replace your cell phone. However, combined with some advanced setup it can certainly enhance your mobile telephony, particularly if you get poor reception on your mobile. Start by setting your cell phone to forward to Vonage instead of your carrier's voicemail service. If your phone is off or you're in a dead zone, you can still answer the call when it rings on Vonage. If you let it roll to voicemail, Vonage forwards a message notification directly to your mobile via a text message (or, for those of you with smart phones, an e-mail with a WAV digitized sound file of the message attached), so you still get notification on your cell. You could send a copy to GMail as well, creating a permanent, searchable archive of all your voicemails.
Using a hybrid cell network/VoIP calling setup means you can make and receive calls almost anywhere on the planet without significant costs or difficulty. Putting a VoIP phone on a keychain further enhances mobility without sacrificing features.