What digit does your extension number start with? I expect you've seen this before:
2xxxx - these numbers cannot be called from outside
3xxxx - precede with a 4 to give 01865 4 3xxxx
530 xxxx - replace 530 by 35 to give 01865 35xxxx
553 2xxxx - replace 5532 by 22 to give 01865 22xxxx
553 3xxxx - replace 5533 by 73 to give 01865 73xxxx
553 4xxxx - replace 5534 by 74 to give 01865 74xxxx
553 5xxxx - replace 5535 by 85 to give 01865 85xxx
7xxxx - precede with a 2 to give 01865 2 7xxxx
8xxxx - precede with a 2 to give 01865 2 8xxxx
9 xxxxxx - remove 9 to give 01865 xxxxxx
(Good grief, you need an Oxford degree just to understand the phone system!
)
Numbers starting 2 can be called from the outside using the 0870 number, I believe.
There is probably a geographical number that the 0870 number maps to, but as it's only one number and could be any Oxford number, I can't see an easy way of finding it without it being leaked by someone in the know!