There are two main ways to do this, depending on whether the number is in your recent calls list or your contacts.
Android interfaces vary slightly depending on the manufacturer (Samsung, Google Pixel, Motorola, etc.), but the core process remains similar. Below are the instructions for standard Android (Pixel) and Samsung Galaxy devices. block or unblock a phone number
No. The caller is not sent a notification. However, astute callers may figure it out if they are always sent straight to voicemail after one ring, or if their texts remain undelivered for days. There are two main ways to do this,