How to implement the download of contacts to a cell phone?

Question

Programming

Hello.
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I remember in 2010 I accidentally found out that the contact details of the phone on the simbian platform are stored in the .txt format, that is, in a text file. It became curious, but where and how are the phones (numbers), phones on the platform android, iOs, windows stored ???
Why do I need it? I want to know if I can, say, store phone numbers in some CRM system, like in an address book, which is in any normal electronic box (e-mail), so that in case of loss, breakdown, data transfer to a new phone, I did it using export, downloaded and uploaded to a new phone, and there is no need to enter all the contact details again. Why else do you need to store data in electronic form, outside the phone - it's trite to print on a printer, on paper, in case something happens to the server or the CRM system. For some, this is a real problem, how to store, synchronize, transfer your data to yourself, at a convenient time for yourself, in one click, even via bluetooth, even download a file. I do not need third-party, ready-made resources, Google or Yandex drives, or dropboxes, nothing that is available, free of charge, or for a fee. Attention! Personal, handmade.
Question:Is it possible to implement this in a bunch of Internet + application + phone? Or in conjunction site + database + phone, or site + file + phone. What is the best way to store this data, in what form? In what file, or in what format are contacts stored on the phone? I don’t want to use third-party resources for many reasons, and I also want to solve the problem with data transfer, once and for all, to rewrite on paper with my hands and pen so that 100% is preserved - the last century. But there is some security and safety. The paper does not run out of battery, in case of an error in the text, the data on the paper will not be deleted, but it can be torn, lost, wet, but if it is stored with documents, the contacts can be saved in the form printed on the printer - for quite a long time. I'm just such a person, emotional and dynamic, I often change my phone, either because I'm tired, or outdated, or because it's broken,

Answer 1

vCard is a popular, human-readable format for this purpose. Supported by modern, and not so, systems.

Answer 2

You can synchronize files, letters, contacts between Win, Linux, Android and ios open source with your own server using owncloud

Answer 3

Not simbian, but Symbian.
Regarding Android, not everything is so simple, as an option, learn the Android SDK , develop your application that will take data from anywhere, .txt files, a website, a computer and write them to the system's contacts using the Contacts API . It's the same with IOS.