If your carrier is AT&T, download the AT&T ActiveArmor app.
This app shows all numbers that have called your iPhone number, whether they were blocked and the reason they were blocked (including those numbers that never made your iPhone recent list, because they were immediately diverted to voicemail). For numbers that are identified as spam, the app allows you to change their designation.
If you have a different carrier, check if they have an equivalent application that shows incoming calls and the reason they were blocked.
I hope this helps!