Just remembered! Had same problem at a large customer with multiple lines and users.

Telco had changed lines from copperPOTS to go through there SLICK(Fiber Mux). It somehow dropped the ringing voltage down to low for switch to make phones ring, but you could see it on the "C" screen.

Flip-Flop line that is for ACD with another line that is not having trouble. You can also compare the voltages on the lines.

I bet it is a Telco issue, but it will be hard to prove.

Did custmoer change Telco vendors recently?

If all else fails, use a BFH.