The letters beside your room number (A, B, C) refer to which bed is yours. Most of the time, the telephones will be labelled with these letters and the one with your letter on it is the desk you use. Therefore, the adjacent bed is yours.
When I moved in the phones were not labelled and I lived in a triple, so even though I was listed as "B" it was impossible to tell which one was which... so I just took the one I wanted