A few companies out there make "Passive Repeater" antennas, where you put
the antenna outside your building connected to one inside, an it relay's
cellular signals in and out for places with bad  reception.

How would I build one of these?  I bought one which was supposed to work
with GSM 1900, and it doesn't do a damn thing.  As far as I can tell, it's
an antenna mounted to a cheap plastic base and that's it.  Other ones I've
seen have a metal pad on the base and you stick one on each side of a
window, but mine doesn't have this.  Maybe that's why it was so cheap.

Jay