Around here we use Slack for primary communications, Google Hangouts (or Chat or whatever they call it now) as secondary, and we keep an on-call list with phone numbers in our main Git repo, so everyone has it checked out on their laptop, so if the SHTF, we can resort to voice and/or SMS.
I remembered to publish my cell phone's real number on the on-call list rather than just my Google Voice number since if Hangouts is down, Google Voice might be too.