Mobile phones have a "ground plane" on the circuit board. The aerial emits radiated waves against the internal ground plane (theory:see James Maxwell).
In order for a stable signal to be transmitted and received a good aerial and ground plane is required. Most mobiles are OK when it comes to the aerial but suffer from a chronic lack of Ground Plane
Because this internal ground plane is small the aerial is of a tuned length and reciever stations or cells have to be located almost everywhere to pick up the low output reliably.