This process is called registration, and is necessary for the network carriers so that they can direct incoming or outgoing calls to the correct phone.
When travelling, (for example), a phone may move from one cell site to another. This is achieved by the phone evaluating the signal at all times, and switching over to the appropriate cell. This is also logged by the networks and is known as a ‘handover’. The handover also ensures a continuation of the call, rather than the requirement to redial every time a new better cell site is found.
