In any case, modern Android phones and modern iPhones do display an indicator when an app (or the system) is accessing your microphone. I do not know if this can be disabled.
It can’t, for certain values of can’t.
On Google certified Android, the feature is required; apps cannot disable it, and there isn’t a UI toggle for it. A phone manufacturer who added a way to disable it would be breaking its contract with Google and could owe money or lose the ability to ship Google certified Android. As for Google’s own devices, “just trust us”. If you have a normal threat model, that’s probably good enough.
If someone very sophisticated and resourceful is targeting you directly, that may not be good enough. It can be disabled with ADB, and it’s possible to run ADB commands on-device. It would be hard to make that happen without physical access to your unlocked phone, but if your adversary is sophisticated enough and the stakes are high enough, it would be unwise to rule it out.










Of what? This is not a rhetorical question. Security starts with threat modeling, and your threat model dictates the precautions you need to take.
If you’re most people, your main privacy threat is advertisers and data brokers. Other comments have detailed how they collect data, and it’s usually “voluntary”. Defenses against this include a browser with good adblocking like Firefox with uBlock Origin, using websites instead of native apps as much as practical, using DNS-based adblocking, limiting or eliminating use of corporate social media, turning off voice-activated assistants, and preferring open source when practical.
It is not likely that advertising companies are activating the microphone or camera on your phone without your knowledge. The legal penalties for doing something like that in most countries would be ruinous for even the largest corporations, and the motivation for security researchers to check for things like that is substantial. If it did happen, the impact on your life would likely be a small payment from the resulting class-action lawsuit several years later.
If you live under a repressive regime that is known to routinely install spyware on phones, you may have different concerns. If an intelligence agency, large criminal organization, or multinational corporation is directly targeting you and willing to spend more money than most people have surveilling you, they’ll probably succeed even if you throw your phone in the ocean.