When I worked at one of the early paging and cellular companies, every company had phones and pagers on at least one or two competitors for the oncall folks - you can't get a call to fix your cellular network when you are down.

Likewise while at Google, they didn't let us put Google Voice numbers down as our oncall phone number. Some folks even had multiple phones on different networks as backups.

I'm sure they have either other SOTA models or open source models around to help if things to completely pear shaped.

They likely also have dedicated and separate clusters for their own use.