The spelling of the word "callister" may seem tricky, but it can be easily explained using the International Phonetic Alphabet (IPA) transcription. The word is pronounced as /ˈkælɪstər/, where the first syllable is stressed and contains the short 'a' sound, followed by the 'l' sound. The second syllable contains the 'i' sound, followed by the 's' sound and the 't' sound. Finally, the last syllable contains the schwa sound followed by the 'r' sound. Therefore, the correct spelling of "callister" is essential for its proper pronunciation.