UPDATE:

They now allow VoIP (eg Skype). 

Made a FoI to the Univeristy and was told to make it to the college.  I will do this.

Is there a separate 0870 number for each room? Or is there only one 0870 number, which is answered by a recording, and you then have to enter the extension number you want?

The latter is how 0870 120 0870 used to work (it was the number callers had to dial to reach a room in college). Have these new 0870 numbers replaced the old system?

If there's only one 0870 number, it's like looking for a needle if a haystack.

If there's more than one, you are in with a chance. Are the internal extension numbers the same as the last four digits of the 0870 number? If not, you should try the internal number with the Oxford codes.

Also, is there an Oxford number allocated to NTL that starts with 0 or 1? i.e. 01865 0x or 01865 1x

Looking at an internal telephone directory it states that room numbers cannot be directly called from outside the Uni's network. 
We've found the best thing to do is to call a random College's lodge and be asked to be put through.  Most will, but some refuse.  If the latter, either abandon  them and try another lodge or give them a story about calling from abroad.