I did a bit of looking. Phone lines, at least western ones, did provide 50VDC. Some rural loops used range extenders at 100 or 130VDC. They also applied 20Hz 90

I did a bit of looking. Phone lines, at least western ones, did provide 50VDC. Some rural loops used range extenders at 100 or 130VDC. They also applied 20Hz 90~150VAC to ring the phone. So those could be DC blocking capacitors for the signal. They could also be for the ringer, where they'd block the DC to prevent burning the coils out, and pass the AC through. You'd need a relatively large film capacitor to pass ~10mA 20Hz 150V with safety margin.