Normally your phone would primarily be connected to the cell tower with the most strongest signal (that is: a cell tower that matches the carrier that you have selected... most phones would be on 'automatic selection of network/carrier' in order to find the closest tower where such a match with your choosen network/carrier is possible).
Without making things to technical, your phone can switch from one cell tower to another (technical term: 'hand off').
I'll try to explain the basics here (no geek or smart guys or nerd additions please, just trying to only explain the basics here in order to keep everybody happy and alive
)
When you drive or walk there is a point in which a 'new' cell tower comes into sight and if you drive or walk closer to that new cell tower, there will be a point where both the old and new cell tower have equal strength.
Theoretically your phone could now switch (been handed off) to that new tower, but that would cause your phone to play a game of ping pong several seconds (or minutes) during the time that both signals are equal or somewhat equal in strength.
Why ping-pong? As soon as you phone switches to 'new' station, the 'old' station (the one your phone just came from) looks to your phone as being the next 'new' station, and because the strength matches (or perhaps fluctuates and is at some moments better then the one you are now on) your phone would jump to that 'old' station and this process would go on and on.
This process would theoretically go on for a certain time until one of the two cell towers is too weak to be considered a worthy 'alternative' for the cell tower that you are currently connected to.
In order to prevent that ping pong game from happening, the handoff process will normally only occur as soon as the signal strength of the new tower has reached a certain EXTRA amount of signal strength compared with your current connection to your 'old' station.
As soon as the the conditions for performing the handoff have been reached, the actual handoff takes place (that handoff process is too technical to describe here, but I'm happy to do some explaining if it is needed for the Arjen case, for now we'll just assume there is a technical process called 'hand off'
).
So to answer your question:
Normally you would always be connected to the cell tower that offers your phone the strongest signal, but as soon as you travel, there might be one or more 'new' cell towers that your phone could be handoff towards, but this will only take place if the new signal and conditions are reasonably better then your current connection.
So you could be walking around, having one of more 'better' or 'stronger' towers in sight, but your phone would only switch to those newer 'better' or 'strongers' towers if the threshold of the handoff point is reached.
Hope this helps to have insight of basic cell tower operation.
Cave