Virtualbox to run a virtual machine with your Linux dist of choice. Assuming that you're in a windows environment and don't want to set up a physical machine.
Your question is lacking precision, even in the comments. "Play with it" still doesn't help understanding what you are looking for.
If you just want to try VoIP calls and see if you feel any difference with a regular (PSTN) phone line, you can use a VoIP client. There are tons of them, some commercial, some opensource. A non opensource one but free is X-Lite from Counterpath. It's well featured and easy to use.
You will also need a SIP line that usually comes (even if restricted) with all major triple play offers from ISPs.
Virtualbox to run a virtual machine with your Linux dist of choice. Assuming that you're in a windows environment and don't want to set up a physical machine.
Install Asterisk to get started with your VoIP server. Then you could use this client to make test calls with: http://www.3cx.com/VOIP/voip-phone.html
Your question is lacking precision, even in the comments. "Play with it" still doesn't help understanding what you are looking for.
If you just want to try VoIP calls and see if you feel any difference with a regular (PSTN) phone line, you can use a VoIP client. There are tons of them, some commercial, some opensource. A non opensource one but free is X-Lite from Counterpath. It's well featured and easy to use.
You will also need a SIP line that usually comes (even if restricted) with all major triple play offers from ISPs.
If you are looking for something else than a client, here is a link that might help find your happiness: http://www.voip-info.org/wiki/view/Open+Source+VOIP+Software It has A LOT of information of various levels.