Hi,
What you need is a 'Click to Call' system, perhaps like the one described by Dean here
http://www.voipuser.org/forum_topic_9971.html
There are several good guides on setting up a vanilla Asterisk system on the net
http://www.asteriskguru.org is one of the simplest, or with a little care it can be done on a pre-packaged system like TrixBox. In either case it is probably a good idea to find out how the basic system works before looking at Click to Call. You'll have a far better idea as to what the acronyms mean then.
Depending on the number of telephone lines you want to run, then branching to the PSTN your Asterisk system will need one or more of the following: Analogue Telephone Adaptor (ATA) <single line>,
SIP gateway <usually start at 8 lines upwards> or a TDM card
http://digium.com fitted with fxo ports <Most handle 4 lines, but 24 lines are available>, or you can go the whole hog and have a multi-channel ISDN card fitted
Can't help with where to get the toll free numbers. But if you are going to use pass-coding then you will only need one