- When you setup a new blog.
- When you submit a request to have your blog unlocked after being (falsely) classified as spam.
- When you attempt to publish too many posts in one day.
Recently, Blogger improved their authentication technique. Besides / instead of using CAPTCHA verification, they now offer us the option of SMS Verification - and for some services, Voice Service based verification. As a first choice, they will send a short text message to a mobile computer, using SMS - and we can reply from the mobile device, or type a reply code into our online display. Alternately, they will call our voice service - and an automated voice will read us a short text message, which we can then type into our online display.
In both cases, they require that we provide a "phone number", so they can contact us. Some blog owners are suspicious of this request - that they should provide a phone number to Blogger, just so they can setup a blog (unlock their blog, publish too actively, ...). Certain that Blogger has a nefarious need to collect their phone numbers, they protest yet another invasion of privacy.
Ignoring the unfortunate reality of the limited reliability of SMS / voice service based verification, the privacy invasion accusations are unfounded. Google only wants to use your SMS or voice based "terminal" as a temporary token, to ensure that you are a person - and not yet another hijacked computer, helping another spammer setup a new splog (unlock a locked splog, over publish another splog, ...).