Most newer cellphones models have specific adapters that connect to PC's. These adapters can usually be used for updating the phonebook on the phone and/or for accessing the internet.
Unlimited voice usually doesn't apply to DATA. To get unlimited DATA transfer on your VOICE cellphone, you will have to pay for an additional service fee by your cellphone service provider. Otherwise, they will typically charge you between $.04 to $.18 per KBYTE of data transferred. This ends up being VERY expensive to browse the web as can make retrieval/sending of some e-mail cost a few bucks each time you do so.
Install another phone line, (install charges vary from $30-$120 for another line, depending on your local phone co.) Pay the extra $2x/month for an additional phone line, or if you can, get DSL/cable modem. If you are too far away from the ISP, consider some other two-way wireless or satellite systems like DirectPC (should now be available). Then you should all be able to share the same connection with a broadband router or internet connection sharing (ICS) setup on one PC.
Good luck!