We did this YEARS ago (9.2), so it predates Environment Manager, but it was still platform SAS alerting using SMS. Simplest solution was to leverage existing email capability and send small email messages to 5559991234@phonecompany.com. That was a simple enough solution for us. Other companies had an SMS gateway that you could message.
The solution really isn't "SAS" per se, but integration with 3rd party solutions.
Caveat: of course, this assumes that whatever is causing your platform failure is also NOT causing your alerting capability to fail as well.. (I have found through bitter experience that lack of bad news does not automatically mean good news..)
If alerting is that time-critical, then I'd suggest a combination of SAS alerting and non-SAS alerting (Tivoli, BMC Patrol, etc) should a critical SAS service fail.