Do you mean you want to disconnect the modem when you receive a voice call ? Why would you want to do that ? Are you rying to find a ''voice call priority'' setup whereby when you are on the internet, you still can receive your phone calls ?
You may need your ISP/phone company to help you set it up. You may also need a special software, and an ISP which support V92 and a V92 modem. A discussion is found HERE. The modem may not be disconnected, but you get notified that there are calls comming in.
You can also ask your phone company to provide a ''hunting line'', if available, where you get more than one line for a single phone number which will of course cost more. Well, not many things are free.
If available, DSL may be be an option ?