If the number 16 is in the same area as the number making the call, an area code usually doesn’t need to be dialed.
答案：(B) being called
現在分詞被動語態 being called，作後位形容詞修飾 number，「正在被撥打的號碼」（相當於 the number [that is] being called 的省略結構）；calling（現在分詞，主動語意方向不合，應該是號碼「被撥打」）、having called（完成分詞，不合語境）、has been calling（限定動詞，不能這樣修飾名詞）都不合。
空格夾在主詞 the number 與動詞 is 之間，你第一個念頭大概是「這裡要放一個動詞」，於是挑了最像動詞的 has been calling。或者選 calling，因為「打電話的號碼」唸起來很順。
先把整句的主要動詞找出來。If the number ______ is in the same area as the number making the call, ...，主要動詞是 is，空格那一塊是插在主詞與動詞之間的修飾語，不是動詞本身。has been calling 是限定動詞（有時態、有人稱變化），放進去會讓一個子句出現兩個主要動詞，先出局。再決定主動還是被動。還原成完整的關係子句：the number that is being called（正在被撥打的那個號碼），把 that is 一起省略掉，就剩下 being called 擺在名詞後面。這種「關係代名詞加 be 動詞一起省略」的後位修飾，你在國中看過它最短的版本：a barking dog（正在吠的狗，主動）對 a broken window（被打破的窗戶，被動）。最好的線索就在同一句裡。句子後半是 the number making the call（撥出電話的那個號碼），用的是主動分詞。出題者刻意把兩個號碼擺在同一句對照：一個在撥（making），一個被撥（being called）。看到 making the call，就能反推空格必須是被動。上一句的 the phone receiving the call（接到電話的那支電話）也是同一組主動分詞的對照。
calling：主動，意思變成「撥打電話的號碼」，與句尾的 the number making the call 撞成同一個東西，整句會變成「撥出的號碼和撥出的號碼在同一區」，講不通。having called：完成分詞，強調動作已完成且早於主要動詞，本句沒有時間先後的問題。has been calling：限定動詞，一個子句不能有兩個主要動詞。
be based in（位於、以…為據點）、make a call（打電話）、receive a call（接到電話）、dial a number（撥號碼）、in its entirety（完整地、一字不漏地）、be in the same area as（與…在同一區）。
An area code is a section of a telephone number which generally represents the geographical area that the phone receiving the call is based in. It is the two or three digits just before the local number. If the number 16 is in the same area as the number making the call, an area code usually doesn't need to be dialed. The local number, 17 , must always be dialed in its entirety.
The area code was introduced in the United States in 1947. It was created 18 the format of XYX, with X being any number between 2-9 and Y being either 1 or 0. Cities and areas with higher populations would have a smaller first and third digit, and 1 as the center digit. New York, being the largest city in the United States, was 19 the 212 area code, followed by Los Angeles at 213.
In countries other than the United States and Canada, the area code generally determines the 20 of a call. Calls within an area code and often a small group of neighboring area codes are normally charged at a lower rate than outside the area code.