A JAA radio license is completely separate from whatever flying license you hold. In fact, it is a completely separate piece of paper. So you can obtain a JAA RT license (actually called the FRTOL - Flight Radio Telephony Operating Licence, I believe) without having any pilot qualifications whatsoever - useful for the people manning "MyLocalField Radio" or "MyLocalField Information".
So it doesn't matter whether you hold an Indian ATPL or FAA PPL or just a drivers license. You can get the R/T license by doing the exam. In fact, I think there's two exams: one for VHF and one for HF. If you do VHF only you get a restriction "VHF only", obviously.
Now whether passing the JAA exam really improves your aviation English, I don't know.