Actually, that policy has changed. I was able just 2 weeks ago to check how many unused days were available on my tickets over the phone. They were only able to check 3 tickets at a time, so if you have more, you will have to call back. It used to be that you could only do it at the park but now there is a dedicated number to call to check tickets. I don't have that specific number, but I started just calling the regualr 407-WDISNEY and asked the person and she gave me the number. If you get a CM who tells you it can't be done, def call back and speak to someone else- it absolutely can be done over the phone now. Sooooo much easier than having to wait until you get there!
HTH!
Lorraine