Why i cant rgister to AAH when my router connect to internet

my enviroment is as following:
There are two computers in my home with the ip addresses are and The has setup asterisk server and the has setup a windows xp runing x-lite. The two computers are connected with TP-Link router. When the TP-LINK is not connected to internet with PPPoe.The sip account by x-lite can register to192.168.1.128 But when i connect the TP-LINK router to internet.the x-lite will cant register to It always connect to by watching Active Ports tool. I know it is the NAT in TP-LINK leading to the result.But how can i configure the enviroment so that they all work well?By the way,can the internet people can use my asterisk?I am sure the http and ftp server can be accessed by internet people,but i am not sure whether the voice comunication will work.Thanks for any good hits.
Best Regards.