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.