Can you connect the series combination of capacitor and earphone to the analog line? For this test, there is probably no need to have the SPA or a phone connected at all. Call in and confirm that you can hear the tones in the earphone. Then place the earphone against the mic on your smartphone or computer and use its native voice recorder app to capture the sound.
As soon as I do this, line becomes off hook and I can hear the dial tone through the earphones.
Putting a 1µF and 0.1µF in series with each other doesn’t make sense. The result is a 0.0909 µF capacitor, but that is probably within the component tolerance of the 0.1µF one.
Also, you want to avoid electrolytics and check the voltage rating.
As typical bell capacitors are 0.47µF to 2µF I don’t think a 91nF one should be able to cause a false ring trip, although it might depend on the impedance of the earphone. One REN used to be 1µF in series with 1.8kΩ.
On the other hand, my headphones read 100Ω per side, at DC, so it might be just be necessary to bulk it out with a 1k resistor, or use a crystal earpiece.
Actually, we are not talking about ring trips here, but about seizes, and they really shouldn’t be happening just from the inrush current of a capacitor, so maybe you did you electrolytics or shorted something else with over voltage.
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