I've contacted their support about whether they allow phone calls and if this is their number. We'll soon find out whether this is legit or not. I very much doubt it is legit and would probably result in the person trying to use social engineering techniques against you to get into your wallet.
Since Bittrex support is flooded with tickets because of the recent account verification enforcing and issues, I think that it'll take quite a while until you'll get your answer.
Until then it's safe to assume it's an overtaxed phone number or another type of scam.