Update: After waiting for a little more than a day, the SMS cooldown is done! I still didn't receive any SMS from Steam tho, but it worked when I changed into a new number... (thank goodness I have 2 phones!)
I don't know what Steam's problem with my old phone number, but hey, at least everything is OK now and the walkthrough is LIVE on Steam! -^.^)/ thank you for your support and patience, littlegal100!