The ring voltage is about 90 Volt RMS superimposed on a 48 Volt DC potential.
The capacitor blocks the DC and allows only the AC to pass. The rectifier bridge rectifies the AC voltage and operates the relay. So the value of R1 is chosen to ensure there is sufficient voltage to operate the relay.
When the relay operates, the contact bypasses the capacitor and so a DC current flows and trips the ring. The relay then holds to the DC current.
The DC current through the relay holds the call.