I have a Samsung Galaxy S9 with Verizon. I have blocked an AT&T phone number. What type of notification does the blocked AT&T customer receive when they call and/or text me?
If you blocked the number via the device, it sends the call straight to voicemail.
If you blocked the number via the block option in your My Verizon account, then according to that page:
What does a blocked contact experience?
Customers who call your number will hear: Welcome to Verizon Wireless. We're sorry the number you have dialed has calling restrictions that have prevented the completion of your call.
Customers who text your number will receive the reply: Message Failed.