选B.整个是一个倒装句,only+介词结构应部分倒装,所以选择Only on.
ring后面接宾语,而whoever与whomever都可作宾语,所以whoever也是正确的,综上选择B项
only在句首倒装的情况.例如：
Only in this way,can you learn English well.只有这样,你才能学好英语.
Only after being asked three times did he come to the meeting.叫了三次,他才来参加会议.