A couple of years ago one of my lads who lived at home had to have some tests at the local hospital, arranged via our local Dr's practice.
A number of weeks later, the home phone rang and I answered. caller asked to speak to my lad. Now, at this time, we were getting plagued with cold calls so I asked 'Who's calling please?' Caller said 'I cannot say.' I replied, 'Im the parent and householder, you can tell me who's calling else Im not calling (son) to the phone, who is calling please?'
Again, the caller refused. I said 'Well, Im sorry, Im not calling him to the phone due to the number of cold calls we get.' At this point the caller hung up.
Turns out it was our local practice who was calling but they decided to hide behind the then Data Protection Act. I worked a couple of times with the Information Commissioner's Office on IT projects and I know, when being asked in a telephone call who you are, is not something you cannot discuss.