It’s probably a real offer; sometimes, Delta will make these calls in advance to avoid needing to manage a small oversale situation on the return, especially if a good alternative had plenty of space. If the number calling is from Singapore, that increases the legitimacy, as the call center that handles many of these things is based there. It’s odd the agent wasn’t aware of the offer when you called back in, but not unheard of. The VM should have had details on how to accept if interested?

If it didn’t come from the airlines it’s a scam!! Never give Anybody your credit card or flight information over the phone

How would a scammer know names and flights? What was on the caller id? I would've checked my app too. Sundays are big cruise ship return dates to Seattle just fyi