Save your money don't buy the passive booster stick on things to your phone some have tried to peddle thinking it would improve their signal it doesn't.
You could try to get a signal booster like this one Wi-Ex zBoost. Then just sit it out in the open and it offers a stronger signal to the tower. The only rub here is if the tower signal coming at you is not strong enough it won't help.
For awhile T-mobile was selling a microcell station you could plug into your internet connection (cable modem) to allow you to make cell calls via the internet (this is not a Skype VoIP connection). These have on the most part disappeared as many smart phones (including iPhone) can connect via WiFi directly and by using the carriers app you can create a connection via WiFi (either a tunneled cell call or a VoIP connection).
If your on the go nothing much you can do here. As you can't wire in a better antenna to the phone its self.