The ONT devices are designed to work with -27/-28 dBm input power level at the minimum, however it works even when the signal is lower than that but ofcourse it can get unpredictable as it's outside manufacturer specifications.
There is no real difference for internet and voice with respect to the optical power level. Your problem atleast now, is not the optical power level. Probably a bad ONT device perhaps. Or check your power adapter of the ONT, it can be faulty too.
hiIn my case there is problem when calls are made from landline to other bsnl mobile numbers. Calls are getting picked up but no voice but bsnl mobile to landline works fine. No problem for other networks as of now. Never tried landline to other landline. We also use Panasonic cordless phone for this connection.