Question: why are iago and roderigo planning to call on brabantio?
Answer: When Iago and Roderigo tattle on Othello for marrying Desdemona without her father's permission, Brabantio rushes to his daughter's room and discovers that she is missing. According to the angry father, this must mean that "the Moor" somehow "tricked" his daughter into whatever the two of them are doing together.