After reading about PhoneLabs’ Dock ‘n’ Talk in a Slashdot thread, I came also came across Telular Corporation. They make enterprise versions of cell to landline converters that are designed to be integrated with a company’s PBX. You could use their Least Cost Routing or other products to make calls to cell phones from your PBX using free cell-to-cell airtime. Another potential big savings in telephone costs.