Github Issue: https://github.com/googlei18n/libphonenumber/issues/1265
Context: India has Timezone map Asia/Calcutta.
The city was renamed from Calcutta to Kolkata in 2001, and thus the timezone should be renamed to Asia/Kolkata.
Other important links:
Wiki also lists this as Asia/Kolkata
Timezone (tz) database also lists this as Asia/Kolkata