You are going to have to switch over to VOIP.

Modems with phone jacks are from the dark ages.

Calls over computer needs a headset and SKYPE and Broadband.

If you need a modem for dial up service they are not expensive.

http://www.walmart.com/ip/USRobotics-56K-USB-Softmodem/21978139

You can use something like Ooma, which plugs into your router an uses a normal phone or wireless phon e system. Cost is about $100 for the box and a few dollars a month for taxes. No charges for US calls. You do NOT need a computer in the setup, so you don't need yours on 24/7.

Magick Jack Plus doesn't need your computer either.  It does need a broadband router however. Same with OOMA

The magick Jack Go device has a power jack so that it can work from WIFI.

http://www.microcenter.com/product/382312/MagicJack_PLUS_VOIP_Phone_Adapter

http://www.ooma.com/voipphone/