The phone call comes after his bedtime. He's awake because he thinks that Karen might come back tonight, but he's pretending to be asleep, because, well, bedtime.
He can't hear the other end of the conversation. He can hear his mother crying, and using some very bad words, and talking about how someone is dead, and how she never wants to hear anything from the other person ever again. At some point she scribbles something down on a piece of paper and angrily shoves it into a drawer.
In the morning he confirms that Karen is dead, and that Wishbone is not coming back. He thinks he cries a reasonable amount about this. He watches cartoons for most of the rest of the day. Then he remembers that he was supposed to do something, if Karen ever died, and that he has a responsibility to see that it happens.
When his mother is taking a shower in the hotel bathroom, he opens the drawer and finds the piece of paper and writes down the number on it in his transformers-themed address book. He labels it ALEX.
The next time his mother goes out to buy groceries, he calls the number.