There's no virtual office concept in India. You need to have a physical office address for trading in India. It is provided by the local municipality office. Then you need to register the business as a sole proprietorship or partnership or company as the case may be. Sole proprietorship and partnership is registered with registrar of firms and companies are registered with registrar of companies.
If your work is good you don't have to have a physical address is a cosmo. Clients will remain irrespective of the address as long as your support system is good. Ensure your response to support queries are fast and your physical address won't matter except if you are planning to sell offline.I already have registration, but using the address of a commercial property in a small town. But that address won't appealing for clients. That's why I'm thinking about getting a virtual address in any of those cosmopolitan cities. Regus shows offices in India, but little expensive as paying 20- 40% extra can get access to coworking space.
If your work is good you don't have to have a physical address is a cosmo. Clients will remain irrespective of the address as long as your support system is good. Ensure your response to support queries are fast and your physical address won't matter except if you are planning to sell offline.