There are pros and cons to each approach.
Phone calls can obviously reach more people, as far more people have a phone. But phone call is based on far more limited data and requires more substantial operation cost.
Apps obviously generate far more data, both true and false. But I think this is a good thing. Educating people where their hotspots are is a great reminder to wear masks and practice social distancing. I would love to know where I should practice extra caution.