http://www.sprintenterprise.com/airave/tellMeMore.html
This product will give you a signal that covers 5000 square feet anywhere you have an internet connection. It costs $100 i think plus $10 a month.
http://www.phonetackle.com/In-Building-Repeaters-C284_category_standard.aspx
These products work very well on multiple phones and could be purchased by your company since everyone would enjoy the benefits regardless of carrier. There is no monthly fee, although they are costly to purchase and require somewhat difficult installation that involves setting up an antenna outdoors and a repeater indoors. Read the FAQ for info!
My personal recommendation:
http://www.phonetackle.com/801247-In...ionsAbove.aspx
Hope this helps!