Typical phone lines should run at 48v DC if I'm not mistaken. As to what an ONT or a cable phone modem may kick out I'm not sure. A multimeter is the best way to tell.
I have a 612 ONT and just put a voltmeter on my phone jack. It read 48V DC, right on the nose.
I also have an HP 6110 AIO and while I don't fax near as much as I used to, I cannot remember ever having a problem sending or receiving a fax since moving from a VZ copper land line over to FiOS in 2007.
We've had Fios for just on two years, and have two fax machines in the house for his and hers sideline businesses. Not a single problem ever (which was a lot better than when we had the copper as we had constantly recurring but intermittent errors then) even with long faxes sent to the UK and EU.
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