[hiatus]

Depending on the team, a phone tree in twilio could do the trick, with calls made down the list if people do not pick up for escalation.

[likium]

What happens when Twilio is down? Same questions for your email, sms and server. Part of the difficulty is guaranteed uptime and PagerDuty is rock solid in that regards.

[blindriver]

Pager Duty uses Twilio.