Any app with android.permission.READ_PHONE_STATE ("Phone calls — read phone status and identity")

All those apps receive android.intent.action.PHONE_STATE broadcasts whenever the phone state changes (new call, etc), and for the incoming calls that intent's data always (AFAIK, maybe some firmware builds have privacy controls for this) contains the phone number.

I understand that, I was really looking for the specific app so I know which to avoid. I use fitness trackers.

You can check the full list of permissions of the particular app you use. Full permissions list is also available in Google Play - there's "permissions details" link at the very bottom (https://support.google.com/googleplay/answer/6014972). Personally, I check this list for every app I find interesting.

If "read phone state" is listed then the app knows who's calling you, in a sense system feeds this data. Whenever the app uses it or not depends on how shady it is.