I am on TPS for my land and mobile no's, I have had almost no calls since it settled in.
I had a few and asked them if they are familiar with TPS and ask them why they are committing an offence by calling me, they then always ring off.
When I got annoyed enough by repeated calls I made a complaint to TPS, after several months I got a letter saying that the firm had been fined (hooray). You have to hold on for long enough to find out who they are and make a note of it, or note their phone no to do this, but I believe that even if number withheld, if you give the date, time and duration of the call, TPS can get the records from the phone co to track them down.
Now all I get is overseas cold callers. My caller display shows "unavailable" for most foreign calls (not "withheld") so I always ask what country they are phoning from. They usually lie and say UK. I say i can see thir phone no and it is not a UK one, they sometimes flannel and sometimes lie. I then like to ask them to hold for a while and put the phone down. More often, I do not answer "number unavailable" calls and let them go to the answering machine.
Recently I had a call from an overseas co claiming to be Barclays Connect. I said I would carry out some security checks and asked them which branch my account was with. They then flannelled and backtracked. So I told then they had failed my security screening and I was unable to talk to them.