Some sort of in-building cell repeater may be sufficient.
The gating item, I believe, is whether you can receive
adequate cellular coverage outside the building (e.g. on the roof).
If so, then a simple repeater should be sufficient.
If not, then a wired micro-cell would seem to be required.
If signal is adequate up on the roof, then
you may be able to construct a purely passive repeater,
composed of a high-gain outdoor antenna, some coax,
and an appropriate in-door antenna located in your
If you want some horsepower, you can consider amplified
cellular repeater systems. (Nextel used to install these
for customers all the time. But they ONLY covered SMR band!)
One example of an active antenna repeater system:
I have yet to locate a dual (or tri) band amplified
repeater system. They would be nice, to ensure that
both 800 and 1900 bands worked, so ALL cell/PCS
phones "would just work". If you toss in the SMR bands,
then all cellphones AND Nextel "would just work".
(employees wouldn't be restricted to using just
one vendor, or one band.)
Also, some 800Mhz-band cellphone companies have purchased
additional RF capacity up in the 1900-band. Depending on
how the cell companies ultimately use the new spectrum,
your employees may not receive minimum call-blocking rates,
or may not be able to access cellular data services,
unless both 800 and 1900 bands are accessible.
Some more links that may be of use: