This obviously won't apply to your iPhone but Android users might find the following helpful. Android has a simple call blocker built-in called "Do Not Disturb

This obviously won't apply to your iPhone but Android users might find the following helpful. Android has a simple call blocker built-in called "Do Not Disturb" mode. A lot of people are unaware that it can easily be configured to only ring for numbers in your contact list. No ring for unknowns but you'll still receive a call notification so you can call back if you choose.