I have recently been receiving the same calls from this number and another one similar (01872240331) Everytime I answer the person hangs up without saying a word, its like there is actually no-one there. If you try ringing it back it is constantly engaged. BT suggested that it was probably a machine making the calls and the company concerned are unaware of this. That doesnt excuse the fact that it is really annoying!!!!
Not sure what to do about it.