Google‘s experimental SMS feature which was released for Gmail Chat back in 2008 has now been removed. This means that users can no longer send an SMS to their mobile contacts via Gmail Chat. Google said users can switch to Hangouts and choose to receive messages in SMS settings.
However, US and Canada users can send an SMS through Google Voice or switch to Hangouts and “continue receiving SMS messages as follow-me (i.e. if you’re offline and receive a Hangout message, you’ll get an SMS message).” The company said that “by streamlining our services, we’re able to focus on creating the best possible products for our users”.
By doing this, it seems that Google wants to transfer its existing users to Hangouts. However, there are many other apps that have the SMS feature such as Yahoo Messenger.
Source: Google Operating System Blog