dhhdjbd not sure where you are from, but in usa you can just buy a prepaid sim and card in any store... (without kyc)
In most countries this is not possible, since there are laws that require any reseller of SIM cards to ID the buyer and register the SIM card to the buyer. In many countries, information about who (name and social security number) that owns a certain phone number is public information, so you not only have no anonymity towards authorities, but simply no anonymity towards anyone.
It doesn't matter really, just don't use phone and SMS, and if you need anonymity, never register anything in your phone number at all (so also don't use WhatsApp and Signal). And use a VPN. Then you can use any cellular provider, and still have privacy. They know exactly where you are, but that is all. Not what you use your phone for, or whom you talk to online. Nor can anyone else.
There are still cases where knowing exactly where you are is a problem too. Eg for domestic abuse victims who have fled from their abuser. They often cannot use phones at all because of these laws, because their abuser would find them. As far as I know, buying unregistered SIM cards from abroad are still legal. But yeah, you never know if the seller is reputable or the service is any good or existing at all.