There's plenty of routers that have ADSL, cable, or even dialup interfaces for connection to the internet.
Then you plug this router into your home network and if everything is configured right all the devices on your home network can get to the internet.
I have a bunch of Netgear stuff. All units required patches before working right. After that they've been fine. Simple for some, a pain for others.
As for VOIP, two considerations apply:
1. Bandwidth
2. Latency
Bandwidth is how many seats you have.
Latency is round trip time. If there is bad weather, over-booking, malfunctions, crashes etc latency will be increased by hold-times or increased to infinity (crashes).
ADSL and Cable have enough bandwidth to handle VOIP. If latency is a problem, either your ISP can fix it or they can't. You'll have to change ISP/technology.
I assume by splitting a cable you mean ethernet hub.
And there's no such thing as a digital signal.
Good luck.