Simple model for a loop antenna for LTspice simulation
Ideally, you'd put an inductor there that represents the inductance of the actual antenna. You would weakly couple that to a second inductor that is driven by a current source.

If you have inductors L1 and L2, you might couple them with a statement like

K1 L1 L2 0.01


somewhere on your schematic.

See Using Transformers in 
LTspice/SwitcherCAD III



After that, I'd measure the RMS power that the coil is delivering to the detector circuit (multiply its voltage and current, take the average). Turn down the current generator amplitude until the circuit stops working, and figure out what threshold level of power this represents.

Whether or not an actual cell phone can induce that amount of power in an actual coil can only be determined by trying it.