saying這里是現(xiàn)在分詞的形式,作定語修飾the call,并不是“進(jìn)行時(shí)”,它表示the call 是主動(dòng)say的
而如果與主語是被動(dòng)關(guān)系,則需要用過去分詞,
例如The call answerd by my mother was a telephone fraud.
因?yàn)槭前l(fā)生在過去,所以用was.同時(shí)have也要變成had,因?yàn)槭情g接引用.
大部分分詞作定語的情況都可以用定語從句來代替,如你所說,可以換成
The call that said (that) you had a mail was a telephone fraud.是的，可以用the call saying xxx was xxx.
The man visiting our school was a CEO.也沒有問題。

visiting這里是現(xiàn)在分詞作定語，并不是一種時(shí)態(tài)。表示那個(gè)（訪問我們學(xué)校的）人是ceo
轉(zhuǎn)成定語從句可以是the man who visited our school was a CEO.

例如放在這樣的句子里，意思就完整了，
I didn't know the man visiting our school yesterday was a CEO.
我不知道昨天來我們學(xué)校的那人是個(gè)CEO。定語從句和分詞短語作定語不是都能轉(zhuǎn)換的，例如，

I talked to Kevin who said you were pretty.不能轉(zhuǎn)換成分詞短語 I talked to Kevin saying you were pretty. 此時(shí)分詞短語saying...的主語會(huì)出現(xiàn)指代不清。

下面的兩句完全無誤。
不奇怪，因?yàn)槌醮我娒娴年P(guān)系 ;)