NTL have registered the following codes in the Swansea area (assuming it's the area with an 01792 code):
01792 53
01792 52
01792 54
01792 61
01792 51
01792 60
01792 42
01792 55
01792 41
You just need to try each in turn.
For example if your daughter's number is 0870 xxx 1234, try dialling 01792 41 1234.
There aren't many to try, but you can eliminate some by entering the codes into Google and see if there are any businesses listed with that code.
For example, a few companies come up if you search for "01792 60" (you need the quotes when you use Google) so this range of numbers probably isn't used for the Uni.
If you find the number, the same code will work for everyone in the university. So make sure your daughter tells the Students' Union and advertises it about the campus!
You are lucky if this works. At Leicester they are using a "national dialling only" number for the university residences, so the geographical number can't be phoned from most phones