Try a 0.47 /250V dc or 160V AC -- Polysterene or polycarbonate type caps.there are BOX type caps in the market. you may recover from your unused or scrap also a cap of similar type.
hi,
The 1uF cap has a capacitive reactance to the 'ring' signal frequency, 10 times lower than the 0.1uF you are considering.
Its therefore possible with a 0.1uF, that you will not drive the opto-tx hard enough to trigger the opto detector.
You could try the 0.1uF on the bench, it will not damage anything within the circuit.
the value has to be designed for delivering requisite current into the opto 's diode. lower the cap value, less the current , the circuit can deliver( due to higher impedance- you already have a series resistor)try out experimentally as suggested by Ericgibbs.