So I guess the author uses a smart phone as a pager, but given his passion for uptime, reliability, latency etc. I wonder if he has experimented with an actual pager.
Unacknowledged pages escalate to a secondary oncaller (e.g. if the oncall is out of range/in a tunnel, under a bus) and tertiary depending on configuration (and then it loops, or falls to another rotation, again depending on configuration). The code and services that do escalations is deliberately and carefully vetted to have minimal overlap with production systems (who's failure they might be alerting people to).
Responders can customize their notification methods (push, SMS, phone, and email) and rules, so you can do things like get a lightweight push notification when an alert happens, and then a phone call 2 minutes later if you haven't acknowledged the incident. Teams get escalation timeouts that forward alerts up the chain if the primary hasn't responded after a period of time.
On the other hand, if you set up an IMAPS account for each person's alert address, and then have their smartphone use that account to check email, reliability is quite high. (Strangely, sysadmins tend to have wifi in their houses and good data plans, especially when the company pays for it.)