A phone call with an intentionally annoying tone. It rings for an extended time, unlike a single notification sound you'd have for email or sms.<p>At my shops it's been common practice to use phone calls to escalate, so this works out for me. If your place doesn't/won't do the same, you may be able to use something like Twilio or a less developer-centric, more consumer-focused gateway to call you programmatically on receipt of an event.