Can't use VOIP + internet at the same time

My parents have BSNL broadband. I have installed a Lingo VOIP at their place. Till some days ago - everything was fine. Last few days - they cannot use both at the same time. So if they are on the internet - Lingo does not work and vice versa. Any idea what may be happening? Any help is appreciated.

thanks,

someone might have changed computer's network settings.
The TCP/IP setting for the network adapter on which modem/VOIP instrument is connected should be set to "Get Automatically".