Is the app open source?

No.

Why not?

My first app, TodoTxtMac, was open source. People copied the entire app and charged money for it in the Mac App Store. Based on the software license I gave it, that was permissible, but it still felt like stealing from me.

Why not charge for it then?

Because I want it to be free of cost.

All the other iOS call blockers I found have a free tier, but require an in-app purchase to block ranges of numbers, which is the only useful thing they could do. I wanted to create a free alternative to those apps.

Will it ever be open-sourced?

I will probably open-source the app when I no longer wish to support it.