Activation date for adding phone service to current services

I added home phone service to my current services this morning and was told that I only had to hook up our home phone to the U-verse box. I received an email later saying that the activation date for the phone was March 22, 2019. Today is March 19th.  I called to get this date changed to today being I needed the services ASAP and was told they could not manually change the date.  Why do I have to wait 3 days to activate a phone hooked up to current services?  This sounds ridiculous when I can purchase home phone services today from bestbuy and have it hooked up in less than 30 minutes.