Caller ID no longer displays Witheld

[Obbig]

There has been a puzzling change of the behaviour of Caller ID on my connection.  Previously if a withheld number called my display unit would show 'WITHHELD'.  Now nothing happens and the display unit just shows the number of the previous caller.  There has been no change in my phone equipment so it can only be a change in the information transmitted down the line.  Anyone else noticed this?

[RSSW]

Update: Voicemail was added and then taken away and I still have the problem i.e., Withheld number calls show nothing on Caller Displays so, for example, if there has been a call while I'm out of the house, there is no indication on handsets that there has been a call.

Not only that, I have been charged for the adding and taking away of Voicemail.