I've been utilizing the voicemail service on my Airtel SIM for over three years now, and it's been working seamlessly. The service is particularly useful when the phone is not reachable, busy, or switched off. However, The issue in india is , where despite clear instructions, most people don't leave a voice message. Instead, they speak after the beep like its a normal call, which can be a bit amusing.
You can even set up a custom voice message, which adds a personal touch to your voicemail. It's a straightforward process, and once set up, it's quite convenient. If you're facing issues with the setup. Its completely free. Most people are not aware of this service. Its only available on airtel i guess.