Maybe that's what the invitation is going to be when you call the number......
I am thinking perhaps it is someone who doesn't have the confidence to approach you outright...possibly for fear of being rejected so, maybe this person NEEDS you to respond even if only for their own sake.
Just think, you could be doing someone a huge service by just acknowledging them.....
I agree though that if you call it should be from a pay phone or untraceable number.