Why can you get cell service in the hallway of the 7th floor of a 21-story building, but not in the elevator or basement?
The first basic coverage solution is to install an omnidirectional antenna in the midst of the elevator shaft [5]. The omnidirectional antenna transmits amplified mobile phone signals from the mobile phone signal booster [5]. The disadvantage of this solution is if the elevator cage is far away from the omnidirectional antenna will not receive amplified cell phone signals [5].  The second and most used coverage solution is to install multiple directional antenna in each of the 3 or 5 floors [5]. Directional antenna points down direction [5]. The directional antenna transmits amplified mobile phone signals from the mobile phone signal booster [5].
In general, cell service is better in buildings which are not block by construction materials such as brick, block, siding, or sheet metal [2]. While these construction materials block cell signals, windows are typically areas where you'll receive stronger signal than behind a solid wall [1, 2]. Furthermore, moving to higher floors can help improve cell signal as you're able to clear obstructions closer to ground level [2].Boosting cell phone signal inside a metal building can be a challenge since steel or aluminum walls are very good at blocking wireless signals [4]. Even if you have a perfect signal outdoors, you might get little or no cellular coverage indoors [4]. One way to improve the cell phone signal inside a metal building is to use a building cellular signal booster system [3]. These systems can boost the signal indoors for one or several cell phones or data devices [4]. They will also improve the cell signal to a hotspot or cellular router to improve your data speeds [4].