Is your phone plugged directly into the OBi100 or are you going thru your house wiring? If you are using the house wiring, connect the phone directly to the OBi100.
You can check the phone port voltages by logging into OBi Expert and going to Status -> Phone Status
Here are the voltages on my OBi110:
Idle:
Loop: 0mA
VBat: 57V
TipRing: 45V
Ringing:
Loop: 1mA
VBat: 76V
TipRing: 11V
These voltages will vary by a couple of volts
Make sure you click on Phone Status when the phone is ringing and not on the silence part of the ring pattern.
I never increased the ring voltage, so I don't know if it will help. You can do it using OBi Expert here:
Physical Interfaces -> Phone Port -> RingVoltage
Max setting is 82
If everything fails, try a factory reset using a paper clip.