Some telemarketing companies carry out number scanning (or ‘pinging’). A number scanning dialler automatically calls a long list of telephone numbers and as soon as it detects a ring on the line the call is abandoned. It does this to find out which phone lines are in use. It has been reported that hundreds of thousands of these calls can be made a day - often at night time.
While the companies who carry this out claim that they disconnect the call so quickly that they won’t actually ring your phone, people have reported receiving these calls - their phones ring just once, or they receive a silent call. Ofcom have ruled that number scanning is illegal.
trueCall blocks number scanning calls.