Old POTS faxing worked, customer went to Cablevision (failed) then back to Verizon (we promise you POTS again ! they said). Fax machine can get incoming faxes, not the computers.
Had client take a reading: 8 volts DC on and off hook, and "almost nothing " ( I was not there) on AC with an incoming fax.

A few questions: 1) What other readings do I need to take to know it's the low AC voltage that's messing up the internal modems in the PC's ?
2) How to fix if it is ? Get an old external modem ? Voltage adjustment on line ?
3)I've been told there is no such thing as POTS any more from Verizon: true ?
4) If I send a note to Verizon will anyone there read it or care ?

AC voltage isn't causing your problem since it's only present when the line is actually ringing in. Since your fax machine works as intended, the phone company will probably tell you there's nothing wrong at their end. Can you perform any diagnostic test on the computers? Do they use modems, dialogic boards? How do they connect?

8 volts on hook would be a REAL problem. I don't think someone understands the concept of 'on hook'.
OR, something is tying up the line, causing the incoming computer calls to hit a busy. Time to drag out the butt set and check at the jack with the modem disconnected.

Manually set the connect speed lower.

Darn, I have to get in the truck ? The new business model just called for being useful on the phone and sending a bill !

I've given the benefit of the doubt to the new customer when he tells me he's tested the line on a number of computers, all of which worked with internal modems and fax software prior to the switch over. None work now.
I'm a computer consultant (networks and programming) but having fun with phones, and learning.
Clearly I have to get in the truck; hope to come back with data; many thanks for the ideas and help!

As much as you don't want to hear it, once a copper circuit gets torn down by Verizon (or any other carrier for that matter), it won't go back in as 100% metallic. While it is possible that they may take advantage of other existing forms of pair gain, the reality is that they are all going to IP.

I know, you know and we know that IP doesn't support analog data, but to the telcos: If there is dial tone at the network interface, then it MUST work.

Don't forget that even if they do deliver the dial tone for the last mile to the premises via copper, I'm fairly confident that there is some variation of VOIP involved in the delivery of the circuit. The biggest problem is that the carriers are shifting local exchange codes from the nearby central office to a virtual central office many miles away. They then rely upon inexpensive transport media to get dial tone to the premises.

In your case, it probably does arrive on copper. The problem is what happens in between.

[all of which worked with internal modems and fax software prior to the switch over. None work now.]

I think you answered your own question there.