Do cells communicate only in binary levels?
Binary responses are associated with switches, which in turn are associated with bifurcations in the steady states of the biological system in question. Varying some system's parameter (such as a cell's signalling strength) cells can switch from 'on' to 'off' (or viceversa) states. As an example, check out Gardner's toggle switch
(https://www.nature.com/articles/35002131), in which this property is exploited in a synthetic system. 

However, cell-cell interactions can be much more complicated, allowing behaviours other than binary responses, such as multistability or oscillations. Thus, upon varying a parameter, a cell's state can, for example, go from a particular 'off' state, chango to an oscillating state and then change again to an 'on' state. Thus not only binary responses operate in cell communications, but many intermediate responses are also possible. For a review in multistability in biological systems, check out: http://m.pnas.org/content/101/7/1822.long

Hope it helps!