Often call centres. Their automated dialling equipment calls many numbers at once. When one person picks up the 'phone, the call centre operator is connected with them and the other ringing 'phones simply ring off. Often you will get to the 'phone just as someone else has picked up. Also, this often explains the "Caller withheld their number" message when you dial 1471.
silent calls can be caused by call center auto dialers, they dial numbers in advance of operators being free to take the call but then when theres no operator ready to take your call you get a 'dead call'
There's also a variant used on mobile phones - A call is placed to a mobile phone - but killed before there is time for it to ring - The mobile records "1 missed call", and the sorry user may try to call the number that called them - normally these would be on a phone line costing the mobile user many pounds per minute.