Site A uses three-digit internal numbers and remote Site B uses four-digit internal numbers. All calls to the PSTN are routed through Site B. What dial plan below best represents provision simplicity, assuming the NANP numbering plan?

A.
Translate all called numbers within Site A to four digits.

B.
Translate all called numbers within Site B to three digits.

C.
Translate all called numbers leaving Site A to ten digits.

D.
Translate all called numbers at either site to ten digits.

Explanation:
North American Numbering Plan (NANP) is designed around a 10-digit numbering plan:

(Sometimes you will see it as NXX NXXX XXXX, which means that the first and fourth digits cant be zero or one)

It consists of 3-digit area codes and 7-digit telephone. For telephone numbers that are located within an area code, the PSTN uses a seven-digit dial plan numbers.

Notice that Site B uses four-digit internal numbers means we need ten digits to access site B from an outside PSTN. Therefore, if people from Site A want to call people at site B and sometimes they just press 4 digits then the administrators should translate the called numbers to ten digits before leaving Site A.