The reason FOSS will always be better is because claims like that can actually be validated and audited. Any company can claim their stuff is E2E encrypted, but you'll never know if that's true for closed source software. Even if they do actually do E2E encryption, you'll likely not know if they're doing it properly and with strong encryption algorithms.
I wasn't asking if FOSS can be trusted, I was asking if Zello can be trusted. The general consensus judging by the comments so far is an astounding no, by nobody has really provided any good alternatives with similar functionality.
I wasn't talking about whether or not to trust FOSS, I was talking about Zello (since it's closed source, which I originally mentioned). There might not be a FOSS alternative, I don't know since I have never needed a walkie talkie app. That's why I didn't mention one and only answered the first part of your question. You're right that it probably can't be fully trusted (because it's closed source).
The client is open, but they use a proprietary server, which is just mumble
If you want to host your own Mumble server, you can use a more open client that's not tied to a commercial server... Like https://mumla-app.gitlab.io/
....
Being hostile towards people giving you good advice, is not a great way to get good advice in the future. The reason people are telling you about the benefits of open source software, it's impossible to vet or prove any privacy claims with closed source software. They could be lying, people lie all the time.
Why not looking for distributed mechanism, which don't depend on trusting central servers or particular instances on decentralized mechanisms, like jami, or similar?