Why phone number has 0 prefix?

This comes with a standard formatter for a country as 0 is national dialling prefix for almost all countries. India with or without national dialling prefix the call work from native dialler and which is true for all countries mostly.

Below given the spec from android, 0 comes with NATIONAL format.
/**
 * INTERNATIONAL and NATIONAL formats are consistent with the definition in ITU-T Recommendation
 * E123. For example, the number of the Google Switzerland office will be written as
 * "+41 44 668 1800" in INTERNATIONAL format, and as "044 668 1800" in NATIONAL format.
 * E164 format is as per INTERNATIONAL format but with no formatting applied, e.g.
 * "+41446681800". RFC3966 is as per INTERNATIONAL format, but with all spaces and other
 * separating symbols replaced with a hyphen, and with any phone number extension appended with
 * ";ext=". It also will have a prefix of "tel:" added, e.g. "tel:+41-44-668-1800".
 *
 * Note: If you are considering storing the number in a neutral format, you are highly advised to
 * use the PhoneNumber class.
 */
public enum PhoneNumberFormat {
  E164,
  INTERNATIONAL,
  NATIONAL,
  RFC3966
}