Why couldn't the kids go to John's or Lorraine's houses to play the phone pranks in The Pigman?
Neither John nor Lorraine have a very good relationship with their parents, and so bringing friends over to use the phone would be impossible at both their houses. John says that they
"couldn't use the phone at Lorraine's...because her mother doesn't have unlimited service,"
but even if she did, Lorraine's mother is very suspicious about all her daughter's activities. If she were ever to find out about the phone pranks, she would make life very difficult for Lorraine, associating all sorts of paranoid interpretations to what is at this point an annoying but comparatively harmless shenanigan. John's house is also off-limits, because, as he says, his mother is
"a disinfectant fanatic...she would have gotten too nervous over all of (them) using her purified instrument."
In addition, John gets along horribly with his father, and as a result of an altercation about phone use, his father has put a lock on the phone to limit John's access to it, after which John spitefully put airplane glue into the keyhole of the lock so no one could use the phone at all.
John and Lorraine pursue their phone prank activities at their friends' houses. On Wednesday afternoons, they go to Dennis Kobin's place, because his mother regularly goes shopping during that time, and his father is away at work. On Sundays they go to Norton Kelly's house, because his father has a standing golf date then, and his mother doesn't pay attention to what is going on (Chapter 3).