Delivery Note Led to Suspected Scam Call

I had a delivery driver knock on my door and leave me a note from A+ Delivery Services giving me a reference number and then a telephone number or website to contact: www.a+delivery.net. When I went there, it said possible scam, alert, or their certificate isn’t correct, and then it gave me a number to call at 1-800-691-0591. When I called them about this delivery being delivered to me, I asked what it was? They said they’re trying to deliver something that requires a signature, but then wouldn’t tell me what it was. I did not give her my information to try to set up some type of delivery because she couldn’t tell me what they were delivering. That just sounds crazy to me! Report this in case of something serious or strange—if legit, they should be able to tell me!