The Stone's hotel room originally is number 904 and after Donna gets locked out is accidentally knocking on the door of room 906 which belongs to Dr. Spaulding and is to the left of room 904. Later, Donna is leaving room 906 (her room) and is intentionally knocking on the door of room 904 (to the left of room 906) belonging to Dr. Spaulding.
The telephone keeps ringing after Donna picks it up in the opening credits.