I am not sure if they provide a "voip" service. They are providing an "analogue" service over VoIP rather than a VoIP service. There may be options but it may be that they are just doing a (sort of) like for like replacement just using different underlying technology.
Of course, the main thing that makes it different is that if the power goes then the router can't provide the telephony whereas the old phone port would (usually) as it was powered elsewhere. Of course if you are using wireless phones then the base station would be down anyway if there was no power.
EDIT : PS - just seen your other post. You could of course move to a different VoIP service altogether rather than using Sky.
Edited by ian72 (Fri 14-Feb-20 16:31:09)