Are you mad? The phone number tells you what phone company to call. In most countries, that tells them your name and government ID.
The phone number is the thing that tells them everything about you.
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There is such a thing as credibility. You can extend trust to others that have credibility. For example, security audits from companies that are credible. Or, you use an app because a trustworthy techie friend of yours says they're safe.
But a prerequisite in all these cases is going to be FOSS code and client side encrypt.