Hi Naresh,
If client is able to use then it simply means they are on the same network at which PI is installed so the port P4 is accessible. Now follow these steps and see the result-
1- As mentioned above maintain host entries.
2. Ask basis team that is PI host and PI message server are at same IP? if so you maintain 2 entries in host one for PI and one for message server.
3. try using VPN or proxy to be on the same network.
4. use Jre 6 update 31 or update 45 (mandaory 
)
5. Go to control panel -> java->General tab->network setting-> choose browser settting if you using proxy in the browser -> if you are using VPN choose direct connection.
6- clear java cache- control panel->java->general->settings->delete files
Please follow and see , it will work
Thanks & Regards,
Vikas