Your presentation implies that you do not feel that the cell phone is consistently needed on the level of safek piku’ach nefesh (a remote but rational chance that it is needed to save a life). You may be taking into account that where you live,[1] many fine Jews would drop everything to help a person in distress, and/or that there is an active Hatzalah[2] organization in the neighborhood. We will begin with your assumptions.
A cell phone is generally muktzeh, falling under the sub-category of kli shemelachto l’issur, as its main purpose is to make phone calls, which is prohibited on Shabbat under normal circumstances. Such an object can sometimes possess a higher level of muktzeh, known as muktzeh machamat chisaron kis,[3] if one is concerned enough about its protection to refrain from using it for purposes other than its regular ones.[4] However, nowadays cell phones are reasonably priced and durable enough that people will use them for just about anything they desire, so the standard rules of a kli shemelachto l’issur apply to them.
A kli shemelachto l’issur may be moved for tzorech gufo (a physical use) or tzorech mekomo (to remove it from a place one wants to use).[5] It may not be moved simply to protect it from damage.[6] In our case, your interest is not in protecting the phone, but in its possible usage in a permitted manner. The questions are: Is it enough that you are not moving the object to protect it, or do you need a positive tzorech gufo or tzorech mekomo? If it must be positive, how exacting are we in determining utility in order to justify moving something muktzeh?
There are discussions among the Acharonim that seem connected to these questions. For example, the Shemirat Shabbat K’Hilchata[7] rules that the need of the place is to be taken quite literally, i.e., the muktzeh object is occupying a place that one wants to actively use. If it is only that the object is an embarrassment or is otherwise unwanted where it is, it may not be moved. In other words, there must be a well-defined need of place. However, not all authorities agree with this view,[8] and the conclusion may depend on the parameters of the case.[9]
The Mishna Berura[10] says that one can move a kli shemelachto l’issur to use it for permitted purposes only if a non-muktzeh article is not available. This again seems to indicate that the need must be significant. Many poskim limit this stringency to cases in which it is easy to use the non-muktzeh object,[11] and not all feel that the Mishna Berura’s reasoning is unanimously accepted.[12] However, there still seems to be an assumption that there are significant standards for application of tzorech gufo u’mekomo.
Our case is special in two opposing ways. On the one hand, the potential usage is the most important one possible – saving lives. On the other hand, the chances of needing to use it on Shabbat appear extremely small. This is relevant, as the gemara[13] says that placing sticks to separate the loaves of the lechem hapanim and prevent their spoilage is not considered tzorech gufo because it is unlikely that there will be spoilage in a short time. This implies that if the chance the object is needed is minimal, it is not considered a valid need.[14]
Tying things together, we suggest as follows. If the appropriate, sensitive health experts feel that there is even a remote but rational chance that the cell phone will be needed to save a life, it is permitted and absolutely proper to take it with you, first and foremost because of the piku’ach nefesh need (although it will also make it tzorech gufo). If it is felt that the chance the cell phone will be needed for an emergency does not reach even that low threshold, then not only would piku’ach nefesh not apply, but there would not be a valid tzorech gufo and it would be forbidden on grounds of muktzeh.