Complete question:

Besides my work as an on call worker for JAM!, I also work at another company. This means that my regular working activities continue. Am I entitled to a partial compensation, since my hours at JAM! have decreased?

I your hours at JAM! decrease and you also work at another company, then you could be entitles to a partial compensation. You can call the UWV to ask about your situation. Moreover, there's more information here: click here for the website of the UWV. We wish you well during this unpleasant period of time.