The use of cell phones is likely to expand even further this spring when all U.S. residents gain the ability to transfer landline phone numbers to their mobiles.

But there's a stumbling block in the path of the cell-phone juggernaut: poor reception inside many buildings that makes cellular networks not quite reliable enough to be the main phone systems for offices-or even for residents of many apartment buildings. So despite the rise of cellular service, most people still have traditional business and home phones.