AT&T Fiber

I installed an NEC SL1100 hybrid phone system in a client's office in 2015.  They are currently using existing copper lines coming from the street to provide five (5) CO lines.

AT&T is now making them switch over to the new fiber network.  The AT&T person showed up on site yesterday and told the cline and myself that the line voltage will drop from 48 volts to 24 coming off the fiber.  He also said that the phone system will not work properly with the voltage drop.

Can anyone confirm if this is true or not?  So for every time that AT&T switches a client over to fiber, their phone system may not work?  This does not sound right to me at all.

Any thoughts?

Thank you.

Jim