Try these two scenarios:
Reception: "Can I ask what it's regarding?"
Me: "It's a sales call, actually"
OR:
Reception: "Can I ask what it's regarding?"
Me: "We sent him some information so we're just following up"
Both are factual and honest, yet one is what people actually want and one is a gentle deception aimed at getting past the "sales filter". I almost feel bad using the slight deception. Almost.