For the tracking of this issue, I don't have a specific link for it that I can provide as it's for an internal enhancement. What I can say is you can check our releases page and look for CAMP-27280 which is the enhancement reference number.
Once it's available, we should have it listed on the page.
As for the content block, typically this functionality is used to add URLs or pull personalized information from the local database. I'm not seeing anywhere in ACS where this functionality was being used to make outbound calls. I am also not seeing any indication of any workarounds in the enhancement (CAMP-27280) which further leads me to believe that this is not possible.
Craig from Adobe Campaign here. From what I'm seeing on my end, it looks like this functionality is in the works and is pretty close to release but is not yet a part of the application. Once part of the product, you should have the ability via a specific workflow activity to make an outbound call.