As per the other comments, that's an incompetent scammer calling. Unfortunately it is easy for a competent scammer to "spoof" the number shown on your handset, so you should NEVER assume that just because the number is one you recognise that it is that organisation calling. The scam callers won;t be impersonating your aunt Mildred, so if the handset shows Mildred calling, it will be her. If on the other hand it is valid number for VM, or your bank, or HMRC, that means NOTHING. Sometimes they'll just use a random private customer number on the basis that people are more likely to answer a UK landline number that doesn't look like the sort of 0345/0800 or semi-distinctive numbers often used for marketing calls.
There are industry plans in place to make this number spoofing much harder, but by the time the painfully slow moving telco industry get round to that, the scammers will have worked a way round the plan. For the forseeable future you're on your own with scam calls, and the best defence (other than assuming all incoming calls are scams) is to buy one of the BT DECT phones with decent options for blocking, delaying or diverting to answerphone incoming calls that aren't in your own list of recognised numbers. The Advanced Phone Z, or the BT4600 are good choices.
