Can you explain that a little more, what service are you trying to call?
Almost the whole model is available as a Rest API call, which means you can call it as a service in a mashup.
I do want to add that some of these services should never be directly exposed in a mashup (like CreateThing or AddThingShape), these types of services should be wrapped (and therefor will reside within a Thing) so that proper security, validation and error handling can be applied.
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