As iOS technology has advanced, older devices like the and iPhone 3GS have been left behind. Running iOS 4.2.1 (the final version for the iPhone 3G) means many apps in the App Store are no longer compatible. If you are looking to install Viber on iOS 4.2.1 , you cannot simply download it from the App Store.
If you have a jailbroken iPhone 3G/3GS, this is the most reliable method. (using Redsn0w or Greenpois0n).
For troubleshooting or finding specific software versions, visit communities like the LegacyJailbreak Subreddit .
If the official store does not work because you have never downloaded the app before, you will need to source the .IPA file manually. The .IPA is the iOS application package. For iOS 4.2.1, the golden file name is frequently cited as Viber_Free_Phone_Calls_Text_v2.2.0.ipa , though stable versions exist ranging from 2.0.2 to 2.1.5.