The iPhone 4 is supposed to have better reception than prior generations. Its additional noise-cancellation microphone should result in better call quality and less distortion during chats. Our somewhat unscientific tests confirmed it - iPhone 4 is go for talk.
We made a series of calls to a regular landline from an iPhone 3G, an iPhone 3GS and an iPhone 4 (with a Bumper case on). We did silly things like read fairy tales, sing songs and discuss bunnies. And while doing these silly things, we rated the quality of the call - mainly paying attention to distortion, volume and any temporary cutting out of sound.
Our ratings were on a scale of one through five - the higher the number, the better the call quality - and were based on what our little ears heard and therefore rather subjective. These were our results once the ratings were averaged out:
iPhone 3G: 3.7 iPhone 3GS: 4 iPhone 4: 3.8
Taking into account how subjective our ratings were, we feel that it's fair to say that the call quality appears to not vary much between the generations of iPhones - though the iPhone 3G did have the most distortion. We believe this could be interpreted as showing that 3G owners will definitely benefit from an upgrade to a newer generation.
Our second test was to check just how good the noise-cancellation feature was throughout the iPhone generations. To test this we played music, a vuvuzela-dubbed video clip, and a speech during phone calls using each device.
While we found the iPhone 3G's noise-cancellation to be terrible and the iPhone 3GS' to be fairly decent, we were blown away by the iPhone 4's. After a few seconds of background noise, the device would be able to filter it out almost entirely and provide us with crisp and clear calls. In this regard, the upgrade to an iPhone 4 is definitely worth it for both 3G and 3GS owners.