You are right, Carola. The perfect continuous or perfect progressive aspect ("I have been verbing") means that an action is to be thought of as filling a span of time, either by happening continuously or by happening in many small occurrences. The perfect aspect without the continuous aspect ("I have verbed") suggests that the verb's action was completed at some point during or at the end of that span of time (perhaps more than once, perhaps only once).
Maybe these examples will help clarify the difference.
I have been running for the last 20 minutes.
⇧ Throughout all of the last 20 minutes, I ran non-stop.
I have run within the last 20 minutes.
⇧ At some unstated time within the last 20 minutes, I started and completed a run.
I have been flying since 9:00 this morning.
⇧ I was flying (presumably on airplanes) non-stop between 9:00 and now. I might have taken a direct flight; in that case, the direct flight lasted from 9:00 until now. Or I might have stopped at one airport and continued with another flight; in that case, the flying refers to all the flights, neglecting the layovers between the flights. The perfect progressive tense asks the reader to think of the flying as filling the whole time interval, even if literally it does not.
The above sentence does not say whether the flying is complete. I could have just arrived at my destination, or I might still be flying. I might even say this on the phone during a layover while waiting for a connecting flight, because the progressive aspect suggests that the listener think of the flying as continuous, and the perfect progressive avoids suggesting anything about whether the flying is complete or not.
I have flown since 9:00 this morning.
⇧ It would be unusual to have an occasion to say this. It means that I flew at least once between 9:00 and now; within that time interval, there was at least one completed instance of flying.
I have been trying to reach you since 9:00 this morning.
I have been calling you since 9:00 this morning.
⇧ Between 9:00 and now, I made many attempts to call you—so many, that you could imagine that my efforts to reach you were like one, long, continuous, non-stop effort. There is no implication that calling you has completed.
I have called you since 9:00 this morning.
⇧ Between 9:00 and now, I placed a call to you, perhaps more than one call. It would be unusual to say this, because if the call was successful, presumably the listener remembers receiving the call; if I made one unsuccessful call, I would probably say when I did it ("I called you at 10:30 but you didn't answer"); and if I made many unsuccessful calls throughout the day, then I'd say "I have been calling you since 9:00."
I have been practicing Bach's Italian Concerto.
⇧ I practiced it many times recently, perhaps once every day or two. You can think of the recent past as being filled with many occurrences of my practicing. Perhaps I am ready to perform the concerto now, and perhaps not. The practicing may need to continue into the future.
I have practiced Bach's Italian Concerto.
⇧ I completed my practice, probably suggesting that I am ready to perform it now (or that I gave up or that I am about to give up now). The time when the practice was completed is not specified: maybe I practiced it many years ago, many I finished practicing it this morning. This sentence could also just mean that I completed my daily practice of the concerto earlier today.