The Keaton household has only one phone line (number) despite having several telephones meaning only one call can go out or come in at a time. Therefore, there would be no way for Alex to call out until Jennifer hangs up: thus, the likely reason for Jennifer's irritation when she realizes that after she hung up in response to Alex's threat to call Adam.
When Elyse is telling Andrew goodbye before leaving for dinner, she tells him to be good for his brothers and sisters. Andrew only has one brother, so she should have said brother and sisters.