Right, I don't know if this will be any help, but i worked for this company for all of about two days, allbeit about a year ago. They are a legit company, although only by being extremely cunning about their business. The call centre, when I worked there, is based in Cardiff, hence the Cardiff 02920 extension. They get numbers from a massive database provided by any third party that could possibly offer numbers to them. Check the next time you sign a mobile phone contract, as a lot of the numbers come from neqw phone contracts that people skim the t's & c's and miss the TINY tick box saying don't give out my number.
The company, again, when I worked there, were licensed by 3G to sell contracts on their behalf. The floor managers used to give us a pitch that wasn't lying as much as bending the truth. The offer would be, for example, an 18 month contract. We'd say, the first 6 months will be 20 quid for loads if minutes/texts etc, and literally be told to not say anyhting about the fact that for the last 12 months the price would double!! The web site etc has changed a LOT in the past year, and they are obviously getting bigger and bigger. My advice, ultimately, is to answer the call, and ask them to remove your number from the database. The computers they use have a facility to do this, although they will tell you it can take up to a week. Threats etc do not make a difference, they get it every day, believe me. Although feel free if it makes you feel better.
Call centres are required to remove your number if you ask them to, try if you can to get the name of the caller when you ask for it to be removed, and if you get another call, feel free to rant away. Hope I have been of some assistance? post any questions here, I'll see what I can do about answering them