call on要求, 请求
例句:He called on his friends to help him.
他向朋友求助。
4.Since then, there’ve been tense negotiations between the opposition leadership here in Benghazi and the British authorities - negotiations which continued on board the British warship HMS Cumberland when it arrived unexpectedly in Benghazi today.
这句话的主干是前半句。破折号后面是进行进一步补充说明。which continued是定语从句,修饰negotiations。when it arrived是时间状语从句。