Its probably an auto dialer.
Telesales companies use computers to randomly dial numbers and if it finds a number that gets answered it will put the call through to a free sales person. Of course if the sales staff are all busy on calls the line goes dead. However the computer remembers that it found a real number and keeps trying at random times until it manages to call you at a time that coincides with a staff member being free.
In theory signing up for telephone preference services should stop it.