Many people do exactly this. Honestly, it is not ethical, and you will probably not last long. Imagine that you are working at a big call center in India. Imagine that your boss has 500 seats and many managers, and that you are one of the managers. Imagine that you have a database of his clients and know a few. You could network and arrange a mini-mutiny. It happens all the time. People get sued over this.
What can you offer this client that your boss can’t? Your boss has overhead. He also has something you might not have — knowledge. Your boss understands the REAL expenses and risks associated with doing business. He also knows that HR costs really are, not to mention rent and utilities. All you understand is that the client is paying so many lakhs per month and that it could all be yours just for offering a better price. After all, you could rent a small place and do the job for half the price, right?
Wrong. You would have to hire workers who might quit the next day. They will not be trained by you. They might deliver horrible performance. Your VoIP system might not work properly which will ensure you will lose all of your clients overnight. You might underestimate some hidden costs. The Indian mafia might want a payoff from you that you are unaware of until they threaten to break your legs. These are things your boss knows that you don’t know.
So, if you do steal a great client and offer them 40% off as a come-on rate with the idea of jacking up their rate after the initial contract expires in three months — beware. You will NEVER get your old job back again. Your boss probably knows others in the industry who will be alerted of your wrongful deed. Also, you will probably go out of business. You will be jobless and broke and deserve it for sabotaging your boss, and endangering the business of his client. You can not service his client like your boss can. The quality will not be the same.
If you want to start a call center, get call center management experience. Preferably at least four years of it. Learn how to network with clients. Learn how to hire and train workers. You need to understand EVERYTHING about the business, not just bits and pieces. I suggest that when you are finally ready to start work — start small with smaller clients and work your way up slowly.
Good luck and be ethical!