I have purchased whatsapp & viber .Both apps are not compatible due to currently I have an OS 4.2.1on 3g .Please advise whether we have this apps supported in OS 4.2.1.A prompt reply it would be highly appreciated
Only the current version of each app is available in the store. Unless you had downloaded them when they were compatiblewith 4.2.1 and you still had a copy of those versions somewhere then you won't be able to use them on your iPhone
You are on the highest iOS version that the iPhone 3G supports, so you won't be able to install the current versions of those apps on it - unless you downloaded and still have a copy of an older version of each app then you won't be able to use them.
The version in the store now needs iOS 4.3+ - unless you had downloaded the app when it was compatible with iOS 4.2 and you still have a copy of it somewhere then you won't be able to install it on your phone