COULD IT BE THE CULPRIT IS TWO PEOPLE?!?!?! :P Anyways...
Haha Im tempted to guess the aunt but... This is why I wanted to guess the aunt: Perhaps the aunt dyed her hair to the colour of the mother... Then she walked by and called out to the boy from the window. The boy thought it was his mother so he opened the window. But to his surprise he realised it was his aunt. His aunt asked the boy to bring the phone along when she saw the phone on the table. Then she called his mum when he is not looking and passed it back to him and gave him some candy to silence him BUT I dont know the motive. And what if the phone was not there?! Okay... so no.
How about Shinte Raku- Hebi Yoshi-san's regular child-sitter and housekeeper? Well, he is the only one who knows that the mum's phone is in the son's room since he is the only one who have entered the room. He could have told the son, or left a note and telling him to take the phone and hide somewhere until a specific time. Maybe he told the boy that there was treasure (candy) to be found and do everything he needs to find the treasure? So the son heeded the note and went to play, and returned at the specific time with the candy. But how did he manage to disguise the voice in the receiver? But anyways, since he called Karasuma as Karasuma-SAN instead of Karasuma-kun, it indicates that there is a sign of respect for him. So perhaps its him. There's a possibility?
How about the childhood friend? Well, the motive is huge, during the time of call, although she is within sight, it is unsure of whether she is within hearing range... Also, the child could have taken the child out already before she arrived and then returned back to the house like nothing happened. She could have asked the child to take out the phone... then she used it to call the mother. Then when they went to deposit the money, the friend possibly hid the phone in the house and placed the note on the money and then used a nearby public phone to call the phone. The child could be nearby the phone and recognized his mother's ring tone and went to receive it. So the friend asked the boy to return home. The motive is as stated above ^