I got an email circular today which suggested that these silent calls are remotely monitoring an automatic dialling system, noting when people actually pick up the phone, to give cold callers a time when they know someone is likely to be in so they can ring again later. This email suggested that if you immediately press the hash key on your phone (#) 6 or 7 times, it will confuse the autodial the other end so it won't be able to track which number you answered on.
I don't know if this is true, but it could be worth a try.