If the ticket is Magic Your Way (bought after 1/1/05) unless it says "NE" (No Expire) on it the ticket is dead. If it is pre-MYW with very few exceptions unused days never expire; some tickets, however, had plus options expire after seven days from first use.
The only way to fnd out about what remains on a ticket is by going to a location which has Automated Ticketing System terminals. This would be any park (including water park and TTC) ticket booth or any Guest Relations Office outside the parks or at DTD.
At those locations not only can they tell you what is still available on them, but if they happen to have become demagetized they can replace them for you.
There is no telephone number to call. Even if you are able to provide a location with the ticket serial number over the phone the CMs are prohibited from releasing this information over the phone. Violation by a CM is cause for disciplinary action, up to and including terminiation.