Internet disconnects everyone a call is made or received

Hi
I have been witnessing a very strange problem with my internet. I have airtel broadband connection and I have two phones at home with the same number. One is a cordless set and other is an usual telephone. The normal telephone is connected to the convertor along with the modem. Whenever a call is made or received via the cordless phone the internet disconnects .I don't face the same situation when it comes to the other phone. I also tried changing the convertor but even that was of no help. Please help
Thank you