SMS service when documents change

Is there a good SMS service that works perfect with Modx that sends a SMS everytime a document (not a resource) in a directory outside the root has changed. I know how to check with PHP when a document changes or when there's a new document but the user connected to that document should get automatic a sms when this happens.

Maybe somebody has experience with this?

It's mostly for clients in Belgium. (in case provider matters)