【英语】------- a mobile phone can you ring ------- you want to

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。