Not sure why this is happening but we noticed it again today. Came home and had several missed calls on phone. Number was a Cardiff number and caller ID showed it as 002920 xxx xxx. Obviously when we tried to call it back we got the number not recognised message. We worked out that the number SHOULD have been 02920 xxx xxx. I don't THINK this is our phones Caller ID display causing this problem as the same number tried to call my mobile (an iPhone) and as a result of the incoming caller info it received it displayed the number as +2920 xxx xxx???
From this it seems that the wrong info is being 'sent' in terms of caller ID rather that the devices getting it wrong.
The best test would be to plug your phone direct in the test point behind your main socket with everything else unplugged, including filters, BB routers etc and see if you are getting the same problem. may also be worth trying a different phone ( or try yours elesewhere)
I might well look at that but it does seem odd that the iPhone, a mobile, gets exactly the same problem. E.g. it sees the number as 00 2920 xxx xxx??