Hi Barbara. The posts you referred to are
here.
This is a Picture House cinema and was listed as unverified in the database.
The main alternative was 01223 328383 labelled "Office; not booking office" and I've just tried it and it is indeed a number for the Picturehouse Arts Cinema, but not the one for booking tickets.
The number you have found, 01223 578939, was down as "administration". If you have booked tickets with this number then that is good enough for me. I have swapped these numbers round to make 578939 the main one.
What you highlight is quite a common technique because businesses often have blocks of consecutive numbers. You may find that other 01223 57893x numbers go to other lines within the building.
Remember though that there is no guarantee that any number is part of a sequence. If you find residential or other businesses nearby then it's likely that the number in question is just an odd one. I seem to recall irrelevant saying that they allocate them in blocks of 10, presumably ending 0 through to 9.
Of course, whilst you may get through to the right business, you don't know what department or room the number goes to. For example, it would be pointless ringing the projectionist to book tickets. Although if you missed a bit of the film, you could ring the projection room and the projectionist could rewind it for you.