In 1984, the telescreen is mandatory for Party members and, as such, telescreens are installed in the homes and workplaces of these people as well as in public areas. The telescreen is a useful tool for the Party because it enables the constant surveillance of the movements and conversations of Party members. It also acts as an effective deterrent against rebellion and thoughtcrime.
There are, however, some exceptions to this rule. Proles, for example, are exempt from having a telescreen and this explains why there is no screen in Mr. Charrington's antique shop. Winston notices this when he goes to visit the shop to see about renting the room above in Part One, Chapter Eight.
In addition, Inner Party members, like O'Brien, have a telescreen but are allowed to turn it off for short periods. This is because Inner Party members are the most influential and important people in Oceania's society, and we see this when Winston and Julia go to O'Brien's apartment in Part Two, Chapter Eight.