I haven't read all of the replies, but I did see mention of Wilson. So I'll post the link to a "large building" [Link: ubersignal.com]

I was also thinking of what Verizon Wireless and AT&T uses. Which are little repeaters that work over IP. Here's the one for AT&T, but I could not find one for VZ. [Link: att.com]

After doing a little research, the Wilson large building may not work legally that is. As this is for a residential use. See FAQ 10 [Link: verizonwireless.com]

Sorry that I couldn't of been any farther help. Though I think the cell booster that I pointed to for AT&T might help. Though you'll need one from them and another from VZ, as they are using different networks. And I would imagine that your client would want one that would cover both type of networks. So his friends/family who visit can receive cell reception.

KOT