I have FIOS digital voice service and a home office where i have a Vonage office number. For more than a year I had an SBC two line phone that worked fine with this setup. It started having some problems, and i got a new AT&T 993 two line phone. There was a hum on that phone. I discovered that the hum was on both lines with both lines connected, but on neither line when only one line was connected. I tried the AT&T recommended trouble shooting--plugging the phone into a separate outlet, moving it away from other devices, etc., with no result. i called AT&T and they said the phone is not engineered for digital lines and I should take it back. I instead exchanged it for another AT&T 993. This one is worse. Still the hum when both lines are plugged in. But also is lots of noise on the Vonage line when both are plugged in. No noise on the Verizon line when both plugged in--just the hum.
I talked to a couple of guys at the local Staples store. One says he has had the same problem, and got rid of his landline phones and uses his cellphone. The other guy said it sounded like too much power is coming out of the digital line. He could not recommend any particular phone for this circumstance. Suggested I try contacting Verizon. I have been waiting over an hour on line and getting frustrated.