Is it mandatory to prefix the phone number with 0?

Please note that dialing 10-digit numbers depends on the carrier trunk being used. However, all carriers support numbers dialed with a "0" prefix, so we recommend asking agents to always use this format for mobile numbers in India, regardless of the carrier being used.

For international numbers, it's important to use the E164 format, which includes a "+" sign followed by the country code and the phone number. E.g. +919xxxxxxxxx