In an effort to block unauthorized use of its Google Apps services, Google plans to require users to verify their identity with a text message if the company detects a suspicious login attempt.

The mechanism, which will apply to logins over the Web, is in effect a form of dual-factor authentication even for those who haven't signed up for that feature explicitly.

Dual-factor authentication requires two steps, typically a password and a code generated by a smartphone app or text message. It involves extra work to log on, but because it increases security significantly, it's arriving at sites including Google, Yahoo, Microsoft, Twitter, Dropbox, and LastPass as a way to better protect accounts.

"When a suspicious login is detected, we send a challenge to the user such as an SMS with a verification code to the user's phone and ask them to enter this code before we grant access to their account. This drastically reduces the chances of an unauthorized user accessing the account because the attacker would have to get a hold of the user's phone as well as the username and password," Google said of the procedure. Those with dual-factor authentication won't have to jump through the hoop, the company said.

Google Apps is a $50-per-person-per-year service under which organizations can grant employees access to services including Gmail, Google Calendar, Google Docs, Google Sheets, and Google Slides. Google is trying to make such services better able to withstand threats such as government surveillance, industrial espionage, and identity theft.

And Google is showing the growing necessity of owning a mobile phone -- and having it charged, connected to the network, topped up with access privileges, and working even when traveling. In effect, a person's phone number is becoming a sort of personal identifier.

Google plans "to slowly roll out this feature for all domains over the coming weeks," Google said in an update on Tuesday. For people who haven't told Google their phone numbers, Google will prompt them to share it if a suspicious login is detected, the company said.

What if a person can't use the text-message authentication for some reason? Google offers a "fallback challenge," and the organization's administrators can temporarily disable the login challenge for a 10-minute period.

Of course, a user might have a tough time flagging IT administrators if there's no phone service and it's impossible to log in to email.