Voice problem over different network(ISP)

Dear Sir,

I am getting problem in voice and problem is as

I am facing a strange problem while using the Xlite.

The Scenario is below:
I installed the Xlite on the two Computers. These two different PCs
are uisng the internet service from the two different ISPs (Internet
Service Providers). Then I register both Xlite to the SIP Server.
The SIP Server is Asterisk Based.
Both Xlite registers successfully on the SIP Server. Then I call
from one Xlite to another Xlite on another PC. Call is
coming successfully but no voice is not coming after call is received.

But When both different PCs use the internet connection from same ISP
then call is also coming from one Xlite to another and also Voice
is coming properly. The SIP Server is on Public IP. But this is
strange that it is on the Public IP and call is coming from one ISP
network to another but no voice is coming.

I also need this customization of Xlite softphone for my business but i am facing this problem please help me …

Please help me to resolve this urgent issue.

waiting for your valuable response…

Thanks and regards

Waiting for your configuration details and logs.