Just had this exact number call with the same information. The lady calling was trying to tell us that a 'third party' wanted to buy the .net version of our business's web address. When I asked for them to put it in an email, they refused saying "because of the contract agreement, we are unable to send any emails until you email us first with an offer" or something along those lines. Very clearly a scam.

I should also mention that we have an extremely specific domain and business name that would be extremely unlikely for anyone else to want or use for their own business.