How successful are you? I assume that the person picking up the phone does not care about the extra booking, nor do they have the authority to offer spot deals.
Not the op, but it's the opposite, at least in the UK. I worked in hospitality as a yoof, was manager of a small hotel and a receptionist in big (independent) 5 stars. I absolutely did care about extra bookings because I got a big commission on walk-ins and a smaller commission on phone bookings. I also had leeway to discount rooms on a sliding scale down to "barely covers the cleaning costs" levels on the night. As a booker if I have the time I still call places up and can usually get a decent discount if they have availability, especially off season or last minute.
Maybe I am wrong and this is a common tactic?