Not sure where this belongs so mods, please move as necessary.

Occasionally I get calls where the caller ID is obviously fake, complete nonsense such as 01111. Yesterday I got new variation, a number that looked like a valid London 020 number except that it was 020 6xxx xxxx. This cannot be dialled (from a BT line, I tried and got "not valid" after the 6) and so was obviously a fake.

Can anyone explain simply how caller ID is faked? Also how some overseas ID get through (had a telemarketing call that showed a valid Phillipines ID the other day) but most do not, including from USA.

There are experts on this board to answer your question.

I'm not one of them but I can tell you that using Skype for all my calls, I can choose the caller ID I wish. So, I presume that marketing companies using VOIP will be able to do the same easily?

OK, a few comments on caller-id from a supposed "expert".

The right to withhold it is considered a vital issue of privacy, that telcos are bound to respect. The actual id of the calling line is held for the purposes of national security etc.

There is no international standard for the presentation of CLID. Because it is therefore unreliable BT generally suppresses it on incoming international calls.

The point about Skype and other voip services is well known. To use a "presentation number", i.e. not the actual calling number, on a traditional network one requires authorisation. It is well known for mistakes to be made in setting these up (the 020 6 case could just be a cock-up).

Some callers use a nonsense CLID simply to get through to those who waste money on the anonymous caller rejection facilities offered by some telcos.

Ofcom considers use of false caller-id, or use of a premium rate number as a presentation number as an example of persistent misuse of telecommunications network or service.

My own view is that CLID can be a useful feature, but it is nothing to get excited about, nor to take too seriously. If you miss a call, you have missed a call. If the caller does not say who they are when a conversation begins or misrepresents their identity then I would see that as misuse of the telephone network.

If you do not want to answer calls from unknown callers get a Truecall box. It will let known callers (based on CLID) straight through and ask others to identify themselves first. If they do so, you will be offered the chance to accept the call by hearing their announcement, or have it rejected by a recorded message.

Thanks for the advice. Got another call from them today and was around to answer it. Somebody with a foreign accent talking about an accident claim. Wasn't terribly clear through the accent whether I was meant to get money because of an accident I haven't had or somebody was claiming from me. I told them to put it in writing or <censored> off. Sounded 99.9999% like some kind of scam.

The number was 020 6203 5290

PS: The Trucall box seems like a very useful tool but at almost £100 (a fact they don't put on their home page) you need to be suffering a lot of phone nuisance to make it worthwhile. Surely some scope for phone manufacturers to put this in their products as a feature.

020 6 shows as 'Designated'

I have no further details on the number range with me at the moment.