call on:拜访,专程的
I will call on you when I am in town.
drop by:顺道,顺便到访
Would you drop by when you are in town?
come:来
Will you come to see me next week?
Would you drop by when you are in town?
你来城里时,顺道来找我吧!(顺道来我家好吗?)
Would you like to drop by (my home) when you are in town?
coming the town 是不对的,不要the,in town是通用的,这儿用coming也不合.