Understanding that the concept behind a cell phone repeater is to get signal to areas that are shielded, I respectfully disagree with Jack.

Things that shield cell phone signals include concrete, plaster lath, metal ducting, earth, water, foil from insulation, and more.

Putting an antenna outside the shield, running a coax into the building, and amplifying and then rebroadcasting is essentially what a repeater mechanism does. If your have the limited wiring in place described, you will essentially be trying to push the signal through a shield. It may work, but it may not. Unfortunately, the only way to find out is to try. Will your customer pay you to try?

I suspect that there may be marginal improvements in some areas, but you really are facing a challenge here. If it's that vital, can you do some retrofitting and get coax inside the building? That will improve your chances.

If you'd like to discuss it further, please feel free to contact me.