These are some tips:

  1. Preparation of phonetic language calls such as altitude, speed, altitude and wind directions.
  2. Manual of radio telephony (102 Pages) by ICAO Doc.9432 will clarify all your doubts about the point no. 1.
  3. WPC website has sample RTR questions. The key to cracking the calls part is precise phonetic language use. Scribd also has sample questions
  4. A detailed knowledge of radio aids, navigation tools, electrical terms, and laws like ohm. faraday.joule. Knowledge of GSM and CDMA
  5. Air regulations for may day and pan calls. 18 Annexes, Various ICAO docs numbers and names. ICAO Conventions
  6. Books from ICAO manuals and R.K.Bail's book on rtr

I hope this helps! All the best with your rtr!

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