All the modem does is turn the packets (the chunks of data) into frequencies that go down the copper to your exchange, where (or close by) there will be a DSLAM (a digital subscriber line access multiplexer) that takes your data and that of all your neighbours and then pushes the data on to the internet motorway.
VoIP puts voice into those packets, which go onto the network just like any other packet. The modem does not know if its handling voice, video or email.
If you are connecting one PC you can plug that PC direct into the modem, and that is connected to the wall socket. If you want to connect multiple PCs then you would normally plug the modem to a router (or buy a combined modem and router, which is more normal now) and then you can plug probably up to 4 PCs into that. Or you could buy a wireless router combined with an ADSL modem and then use as many PCs as you like around the house without any further cables.
If you like trailing leads,
this would be fine, whereas for wireless you might choose
this.