There are a few LINE port ring detection related parameters --
go to: Physical Interface -> LINE port -> Ring detection ->
RingFrequencyMin, RingFrequencyMax, RingThreshold, RingValidationTime, RingIndicationDelayTime, RingTimeout, RingerImpedance.
You may choose lower RingThreshold to detect ring with lower ring voltage ...
If the ring burst is fairly short, then, shorten RingValidationTime to like 100 ms...
Other parameters you might just keep their default.