Hi adren lam,
You fail to mention if you are talking about Mobile Apps or Web Applications, and that is quite relevant here.
For Web Applications:
Up until OutSystems 10, the platform had built-in server-side support to send/receive SMS (you would need to create a new SMS Flow, just like you can create new UI Flows). This would require an SMS Gateway properly configured (instructions detailed in the OutSystems Platform Server's Installation Checklist).
From OutSystems 11 onwards, I guess your only bet is to use existing online services that provide that functionality. You have several available on the forge: Twilio and SMS Carrier where easy to spot there. There are a few other specific to countries like Zenziva SMS for Indonesia, or SMS Talk for Brazil.
For Mobile Applications (OutSystems 10 onwards):
For Mobile Apps, this kind of functionality can still be achieved server-side, but for devices with SMS capabilities you may be able to use the devices' hardware to directly send/receive SMS from your applications. From what I could see, the current forge plugins that support this feature are only available for Android (but you can see here there are other Cordova plugins that are cross platform, but they are lacking an OutSystems wrapper plugin): SMS Cordova Plugin and the more recent SMS Receiver Plugin from the same package author.
Hope this helps.