Signal targeting a cell phone
What makes you think that the signal must be delivered 'exactly' to your cell phone? The transmitter just shouts loud enough that all phones in the cell receive its signal. It is up to each phone to sift through this signal and pick out the parts that are intended for it.

There is indeed an interesting problem for the towers and the phones when an active phone travels from the area of one tower to another one, this is called 'handover'. Basically the towers communicate and decide on the basis of signal strength when the next tower takes over the phone.