It's all too easy for the timewasters to circumvent the TPS. My best defence these days is Call Recognition - I simply don't answer to withheld numbers, overseas callers etc. They can leave a message if they're genuine.
Call recognition doesn't always work though. Anyone ex directory or phoning from somewhere with a big phone network, like the council or even your bank, can be shown as number withheld. I also had a call from a lady in Newton Abbott once (at work) that bizarrely showed up as International. Most firms get round the TPS thing by using overseas call centres or just plain ignoring it or claiming you filled in a survey or something authorising a sales call from them, having said that we have TPS at home and work and it really has cut down the number of UK based sales calls.