We have several buildings at work with signal boosters in them but all costed big bucks from whatever the cellular carrier was.

If you don't care about having your phone on you I have a unit at home that has normal phone plugs on it and connects with your cellphone with bluetooth and lets you use regular phones. You can leave that and the phone where the signal is good and use a cordless phone with it where the signal is bad. Not the best solution but for the $25 or so it costed it works.