If you use a carrier's blocking service (e.g., AT&T Call Protect, Verizon Call Filter) or a third‑party blocker (e.g., Truecaller), open that app and look for a or Spam list to remove the number there.

Here’s a draft guide for unblocking a number on Android:

Are you tired of receiving unwanted calls or messages from a specific number on your Android device? Unblocking a number on Android is a straightforward process that can help you regain control over your phone's incoming calls and messages. In this review, we'll walk you through the steps to unblock a number on Android and provide some additional tips.

Depending on your device and preference, there are several ways to manage your blocked list. Block or unblock a phone number - Google Help