VoIP is Voice over Internet Protocol. It's how Altice's telephone service works, which is thru the moden. It's not POTS or plain old telephone service, which is thru copper wire. You lose dial tone whenever you lose power, with VoIP, because the modem needs electricity to operate. I always told subs or customers who had an emergency lifeline system to go with Verizon's telephone service because that service stays up and running even with no electricity. Hope this helped.