Problem when changing the bindport for IAX

Dear All

I need to change the IAX port from 4569 to other port ( ie. 9876 ) cause my internet provider block 4567.
When doing this IAX client could register.
I use Zoiper Free Edition to test this and also change the IAX port in Zoiper to 9876.
The request from Zoiper will be sent to Asterisk but asterisk doest not provide service for it.
I have checked the receiving request from Zoiper to Linux ( Asterisk Server ) with tcpdump program.
Also when I check from my local network my IAX client could not register when the bind port of IAX is changed to 9876.

The output of my tcpdump is something like this : is the wan IP address of the linux server.
and is the IP of the IAX client which want to register.

09:07:57.439927 IP > UDP, length 21

after that the IAX will response Timeout.

If anyone could help me why still after changing the Bindport to 9876 the asterisk will receive on 4569 ( in tcpdump ) I will appretiate it.

Best Regards,