mr.chen is their history teacher是什么意思?

陈先生是他们的历史老师!
星期一早上他们有多少堂课?
再问： please ask him to give me a call before then.and my telephone number is four five two eight seven nine six是什么意思？
再答： 麻烦叫他在那之前给我打电话，我的号码是：4528796.