A-ha! Why, yes I do use a cordless phone! This could be the reason. Thank you.

Vonage uses a receiver that is daisy chained (modem to computer, computer to vonage, vonage to router) and uses the internet connection that way. so I wondered about it stopping the connection at the receiver when the phone rang. But the cable just had a phone jack in the modem box, so it's probably the cordless phone.

Thanks again... pd