Please guide how to incorporate additional digit(s) in already stored phone number. Example - all those constituents based in Karachi previously had two digit area code '21' are now require to replace with three digits code '201'.

You can also run a validation on the records before you actually do an import. That should spit out a list of your exceptions. You can then run the import of the successful records and then deal with your exceptions. There's a gigantic document on importing - not sure what version you're running...