Not login my sip account from broadband connection

I have my asterisk 1.4 server in my home, working fine. Delhi,India.
i am using 220 bx modem from beetel.

Issus is , when my friend want to login in my asterisk machine as a sip clinet using xlite soft phone remotely.
He can login successfully in my asterisk server, but we are not able to talk each other.
I am using my airtel broadband connection with DMZ feature on my modem.
when i tried to look in issue, i guess there is issue with Rtp port 10-20k.
may be these ports are blocked by my ISP , Airtel.
Now please help to resolve this issue, tell me how can i manipulates the port.

can i change in rtp.conf file in asterisk.
please help

rtp.conf on Asterisk, for the Asterisk side ports.

SP Account, topology on X-Lite.

Both should have been easy to find.

Incidentally, I suspect the real problem is the NAT configuration on both sides, and possibly a router trying to be too clever.