Assuming a landline, this is very strange. Please post any "special" details about the line, e.g. part of triple-play package, derived from ISDN, local PBX extension, etc.
For the Line port, try setting RingValidationTime down to 150 and/or setting RingThreshold down to 19.35-23.65.
If no luck, under PHONE & LINE Status, report the values of State, LoopCurrent and TipRingVoltage, when the line is being rung.