While the statement at the end of the description says “does not support cable digital voice service” that should not be confused with VoIP telephone service (such as Vonage), which it quite definitely does (I’m actually using one currently and have been a Vonage customer for about 4 years).
Many cable companies with the switchover to a fully digital TV system, found they had enough bandwidth that they could also supply a digital connection for a “voice telephone service device” but chose to not use the existing VoIP protocols. Their digital voice is like having a second digital TV signal, and pretty much requires you to use their equipment.