I'm with Virgin media and recently purchased a new phone / fax machine which has caller display built in.

Virgin have turned caller display on, but every call that comes in is "unknown" no matter if its BT, Virgin or a mobile.

Can anyone shed any light?

Is this the only CLID phone you have?

First try a few others that are known to be dual standard.

In most areas cable uses a different standard of CLID than BT.

Could be the phone / fax only has BT version.

Cable uses a Bellcore variant of CLID, BT uses ETSI FSK with PR (UK).

So really need to find if VM have correctly enabled CLID, so try a few known working on cable lines phones.

Thats ridiculas and theres no need for it.

So if someone on Virgin calls someone on BT who has caller display it won't work?

They need their heads banged together. Should be a universal standard. I might report this to ofcom.

No, the caller display will work BT to cable and cable to BT.

What it means is, the vast majority of devices (phones) were made for 'BT' version of caller display and not cables version of caller display. There are more and more dual compatible caller display versions being made.

So if you buy a phone that only has 'BT' version of caller display then it won't work for caller display in the cable areas that use the 'cable' version of caller display.

Same that if you bought a phone with 'cable' only version caller it wouldn't work on BT lines for caller display - though no cable only caller display phones get sold in the UK.

Some cable areas do use the 'BT' version, most don't. Some areas of cable don't even have caller display available.