funded by three-letter agencies

That for sure can feel iffy but if the code is sound and the keys are stored on client only, that does not have to mean they can snoop. Leveraging access might be a vector, though.
The agencies do need something that is a. really secure and b. present in global population, so it’s not a dead giveaway “that guy is a spy”

using your phone number

They haven’t dropped that yet?