Server monitoring is key to the availability of many business systems so knowing as soon as a server or services is having an issue is critical.
It's also not just the engineers that necessarily need to know. Other stakeholders like support personnel, account managers (who are going to be taking customer calls) and even key customers can all benefit from being kept in the loop about issues.
Most system monitoring solutions can send an email in response to a monitoring event. Just wire that up to an Email SMS service and you can notify anyone, pretty much wherever they are.
However there are times when SMS is not enough. They can be missed. If there are key people that you need to notify and need to know that they've got the message a Voice SMS is a great option.
Using IVR menus you can have the recipient acknowledge receipt of the message. It can keep ringing them until they do or your escalation rules kick in contact someone else.