Officially, you have one of two options depending on which model of the hub you have.
If you have the SmartThings/ADT security panel, then there is an ADT option, no long-term contracts, and it’s a decent low-cost security system. It has cellular communication to the monitoring center, it has its own battery back up, and uses a proprietary communication protocol for those devices which are allowed to trigger calls to the monitoring center which should be more reliable than the regular SmartThings sensors. It can also do all of the same home automation. If you follow any of the links on the SmartThings site to “security“ they will take you to this model line and explain all the details.
The alternative for those who have other smartthings hub models is to use the scout monitoring system. Again, no long-term contracts. However, unlike the ADT model, this does not do fire monitoring, only “intrusion“, and does not have cellular communication with the monitoring center, so if your Internet or power go out, it won’t work at all. It doesn’t really make sense when compared to the competitors, but it is available.
Just speaking personally, I would not rely on smart things as my only security system unless I were using the ADT model. But obviously that’s up to you.