This happened to me multiple times when I was still flying UA. On one occasion, not only did I lose a LHR-ORD upgrade because I didn't call back within 24 hours, I also got an earful of attitude from the agent I spoke to when I did return the call. She thought it was reasonable to expect me to check my voicemail/e-mail at least once a day from Ethiopia, where I'd been for 3 weeks!