You need to borrow a modern phone with a caller ID display (from a friend, family member, neighbour) and test your line to see if caller ID is still present. Trying to phone VM support is likely to only confuse the situation as they might tell you any random thing, which may not be correct/true if an appropriate solution does not appear on their on-screen script.
Once you have tested with a modern phone with a caller ID display you will know if it is present on the line. If it is not, you need to get it re-enabled and re-test your caller ID unit.
If caller ID is present on the line, and your unit is not detecting it, this could be due to the age of your caller ID unit.
Going back in time, the UK telephone network used a dedicated wire in the master socket for the ringing signal (referred to as the 'bell wire'). Over time, as more telephony equipment used in the UK started coming from overseas, the need for the bell wire has fallen out of use. Some older UK equipment, however, might still expect to use the bell wire. From topics on here the equipment that is affected seems to be from around the early 2000s and late 90s and before.
Some VM customers on here have reported a fix for the bell wire problem by buying an adapter with a ringing capacitor in it. This provides the third bell wire connection from within the adapter.
So test the line with a known good/modern phone which has caller ID on it first. The you can decide what to do next.
