ADSL modem, I have a question though about voip, can all of them do it?
Absolutely NOT!
A bit of clarification is required. airborne_artist is correct in saying that
The modem does not know if its handling voice, video or email
. However, you must have software / hardware in place to create VoIP traffic in the first instance! Many solutions allow you to use a PC with appropriate hand / head set to act as a "softphone" - e.g. Skype. I use a softphone via VPN to my company's PBX. In these cases, the modem is just passing packets out to the network.
You can also buy devices that sit on your home network and allow you to connect standard phones to them - the devices act as hardware "gateways" to a VoIP phone service (e.g. SIPgate). Again, the traffic flowing in and out of them is passed out to the internet by the modem, which doesn't care what the packets contain.
You can get ADSL modems that combine this VoIP functionality with their role as ADSL modem. Usually you can attach 2 standard (POTS) phones to them, and one of the VoIP ports will fail over to the standard analog PSTN if the internet service fails, or there is a power cut.
is it really fine to get a single port modem and just split the Ethernet cable?
You CANNOT split an ethernet cable - you must install a hub or switch to provide more than 1 downstream connection from a single upstream port.
HTH
SD