In reply to Bob.

I had the Citi desktop app and used virtual card numbers for years. I loved it, but several years ago it became really buggy and I finally gave up out of frustration. I realize that I can log into my account on Citi's website and get a virtual number, but that's a bit more of a hassle than it's worth. It's been a while so maybe some day when I'm up for the aggravation I'll try downloading and using the app again. I did feel more secure when I was using them.
It's also worth noting that virtual numbers can only be used for on-line charges so it's not likely to do you much good if you are signing up for a gym membership.