The alternative would be to build an entire phone, like the Blackphone, but that seems like a hassle and then you can't use your favourite mobile brand, whateve

The alternative would be to build an entire phone, like the Blackphone, but that seems like a hassle and then you can't use your favourite mobile brand, whatever that might be. For any end-to-end encryption project to succeed, it would need to be all of the following: a) very cheap b) open-source hardware + software + PCB layout c) compatible with all existing mobile phones d) easy to build your self using standard components