Resting cellular calcium concentrationWatch
Do you talking about nerve cells?
Well, as you surely know the calcium ions are inside of such a cell, while the sodium ones are outisde of it. As long as these ions are not changed from outside to inside and vice versa, the charges are in an equilibrium (no potential difference), that is to say no stimulus comes into being, so the nerve cells are resting (called resting potential). But when they are changed, the nerve cells are in action and begin to convey a stimulus in form of an action potential. As you can see, it depends on calcium ion concentration inside of nerve cells itself whether they are resting.
Don't know whether its that what you want as a giving answer, but I give myself a try. The resting potential is about -40mV, to get the an action potential, the stimulus has to overcome a certain threshold, 50mV up to 60mV. That is the voltage which has to be achieved by calcium ion concentration (outside of the cell) during signalling activation, so to initate an action potential.
EDIT: also there are many different ways to initiate calcium signalling (e.g. plasma membrane channels opening and letting extracellular ions in or release from intracellular stores) and many different reasons for initiating signalling (e.g. muscle cell contraction, hormone secretion, synaptic plasticity etc.) so there is no fixed value that calcium concentration increases to during calcium signalling