We use a variety of escalation techniques. As mentioned down thread, pagers are actually very unreliable. Some SREs carry pagers and mobiles. Most SREs carry phones with escalation via SMS, an actual telephone call from an automated system (after a delay), and/or escalation via a network connection over the data network. Phone calls are way way more reliable than pagers.

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).