You're right in your assertion surrounding the userTasks; they're where user input, and user interaction in general are located.
I think, in my mind at least, interacting with external services makes a lot more sense when done through a lower level than the UI components provided in something like the designer. I don't think that was your intended attempt, but what I really mean is that implementing these features within Java, then wiring them with service task would be a more optimal, and much more modular utilization of the available resources.
For example, relating to your SMS connectivity:
There are several Java packages available, some of them are open source, and some of them are packages meant to integrate services like Twilio - which is paid messaging platform. Either way, plenty of options - so you have the ability to develop that however you'd like to, following standard Java patterns. Then, within Activiti, you can simply make a call to that Java code, via a service task (there is a 'Class' field in the Service Task, that allows you to call any custom class, and you can specific inputs, etc).
If you could expand a little bit more on the various bits you're interacting with and having to get nitty-gritty with, I can probably elaborate on those a little bit 
EDIT: I didn't notice your original question in the title. I'm not sure I understand exactly what you mean, however, there are REST endpoints built in that allow you to query for variables inside of a task, and for retrieving values in form-fields. Check out the API calls here, in the developer guide - maybe those are along the lines of what you're thinking? If not - we'll dig deeper.
Hope this helps as a start,
-JEarles
bp3
