How to configure an DID number to an Extension mummer is Asterisk ?

If you have to DID number pointed to the IP address of your Asterisk PBX, and want to direct this DID numbers directly to an extension number:
for example, DID number is : 416 1234567 and your extension number is 8888, when dial 416 1234567 from PSTN, this will ring your extension 8888.

in file : extensions.conf
add following line:

exten => _4161234567,1,Goto(8888,1)