The following updated info on calling non-geographical numbers using the pre-dial geographical call through service is mentioned on niftylist forum.
List of geo call through numbers:
01635 578 107
01635 578 562
01793 23 82 80
Calling Non geo number availability:
Weekends** | Evenings | Daytime | |
0044870* | YES | NO | NO |
0044845* | YES | YES | NO |
0044800* | YES | YES | YES |
*You must use the full international format when calling all UK numbers.
**Midnight on Friday to midnight on Sunday [Updated 22/4/2008].
Note that as mentioned previously the numbers can be used to call some destinations abroad in addition to calling the non-geo numbers above. My previous post on these numbers can be seen here.