By default, the system checks the caller ID, and allows you to logon if it matches with the original caller ID used when the account was created.
For better protection, you can set a passcode in the settings menu, and ask the system to check passcode while logon.
By default, you can't logon to your account from other phones. If needed, you can enable it in the settings menu. When you logon to your account from other phones, the system will ask you to provide your own phone number and your passcode.