This is of little interest to 5-eyes.

For a start, installation of the app is voluntary so if you're planning something you really don't want the government to discover, you wouldn't install it.

Also, intelligence services already get call metadata so they know which mobile towers you are using and probably lots else.

As for effectiveness, I think it will be useful. There have been several papers crunching the numbers and it looks like it will have an effect. It doesn't need to be perfect - any non-trivial reduction in R is helpful.