Some friends are just like family,there is no need to make a formal arrangment for a visit,except the very close friends and relatives.Making unexpected calls should be avoided as much as possible.The unexpected guest is welcomed,but whether the welcome is sincere or given out of a kindness is another matter.Pratically everyone has his own plans as to how to spend his time,and it is often perplexing if not an annoying to be abliged to change one's plans by unexpected visiters.when someone invites you to drop by anytime, they really mean to call first to make sure the time is convenient.