There was a time, several years ago, when SMS notifications were very handy. Twitter, Google Calendar, and many other services, used the 160 characters to let you know of any changes to your schedule, new mentions and direct messages, and surfaced other useful information to you. But in today's age, where smartphones are so ubiquitous and you could get one for less than $100, SMS is taking a step back to regular notifications from installed apps. That's why Google is canning SMS notifications for its Calendar service on June 27th.
Important: SMS notifications not available after June 27th
Starting on June 27th, 2015, SMS notifications from Google Calendar will no longer be sent. SMS notifications launched before smartphones were available. Now, in a world with smartphones and notifications, you can get richer, more reliable experiences on your mobile device, even offline.
Note: This change will not affect Google Apps for Work, Education and Government customers.
Google's recommendation is to use its Calendar apps for Android and iPhone (other OSes are left to dry, unless you sync your calendar to their built-in one), which will be able to notify you in greater details and more accurately, even when you don't have a signal. Google Apps for Work, Education, and Government customers will still have access to SMS notifications, but I wouldn't count on that being the case for a long while.
- Google Support