Cell Phone Antenna

A Yagi with a Velcro stick on may solve your problem.

Check with your cell phone service provider for any recommendations they may have. They probably won't but it never hurts (much) to ask.

The Yagi antenna proposed by Hendrik should meet your needs. In order to orient this, you will need to know where you are getting your strongest signal from. The closest cell tower may not be the actual one. You can do a sort of RF survey yourself using your cell phone: Go around the outside of your house and see where you get the strongest signal -- most bars. Take a reading every 3-5 meters by waiting there at least 10 -15 seconds. This will give the handset time to register the strongest signal at that location. Once you know the direction, you can orient your Yagi.

If you want to be even more precise in your RF survey, go to wpsantennas.com and download the Field Test Modes pdf (under More Info on the left column). This will give you information on how to put your handset into a field test mode which will give you a precise signal reading. What you want to look for is a Received Signal Strength (or RSSI) that approaches 0. Anything above -65 dBm is going to be very good. Anything below -105 or -100 dBm will not give you sufficient signal.