Call your service provider. If your phone is connected to a modem then it is likely because your phone service is being provided through your cable company, and your phone line goes through an internet connection in order to work. If you do not want to connect directly to the modem then you should be able to plug the modem into any home phone jack, and then plug your phones into any of the other phone jacks in your home for normal use.
Another thing that can be happening is that you have some form of DSL internet service, which requires DSL phone/internet filters for each device plugging into a phone jack. The only way to get around this would be to move your internet service to a different provider (or medium), cancel your DSL, and then you should be able to use your regular phone wall jacks like normal.
Either way you will need to contact your provider. They should be able to help you with this.