Cold storage is where the secret keys are kept offline so that they can not be stolen or exploited through the internet. With a hardware wallet, the most common method of storing private keys is offline. Although hardware wallets are the most common technique, they are not the only method; a smartphone is one substitute. You can definitely trust the platform BitAddress.org for transactions.
There are three main reasons why you’d prefer a smartphone over a hardware wallet. Hardware wallets are often told to return. Also they are not easy to get at all times. You may already have an old smartphone around you and you don't want to look for a decent device with your money. As you can load several wallets on it and thus store a greater variety of coins, a smartphone offers you more choices.

Prepare your phone

To start the process of making an Offline crypto wallet, follow these guidelines. If the device, remove the SIM card. You don't want information transmission at all, just WiFi. Perform a refresh for the factory. This is usually found under Settings/Backup & Reset. This method wipes the computer of all private information and returns it to a factory state. Back up something beforehand that you need.
Moving on, Build a new account with a false name. Save the account info for later on. For almost nothing but tasks related to communicating with this system, this new account should be used. Do not back up your device to Google's servers during startup, do not want to remain up to date, and do not add any billing information during setup. Now you have set up your smartphone for cold storage, store any cryptocurrency like bitcoin or ethereu etc.

When it asks you to secure your phone, choose PIN and make sure it’s at least 6 digits long; I use 10. Remember, this isn’t about convenience, it’s about security. Choose to not show notifications when the device is in lock mode. Lastly, don’t enable any of the offered services like backups, location, bug reporting, etc.

Encrypt your mobile, at last. On the Security tab, there should be an alternative. Your protection is a top scorer and isn't worth taking the risk if you can't encrypt your computer, and you'll be better off using another one. You can use platforms like Coinbase for cold storage.