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We will be asking our users to generate an account verification code that will be texted to them in order for them to sign in (as an alternative to emailing a password reset link). How many digits of the (mainly UK) telephone number can you show (so users can tell it's the right one), without compromising security? I have seen this on other sites:
07*****1234
A general principle to follow is to provide enough information for a user to identify the known phone number, while restrict others from any unintended use of it.
Most commonly, last two digits will be enough to recognize a phone number.
Let's have a look how a verification may be implemented.
Google provides just two last digits:
Yahoo shows two last digits, and the first one:
It is notable that Yahoo page itself named
"Phone obfuscation challenge"
that clearly reflects its purpose of hiding the phone number from an unwanted use.
