[Asterisk-Users] Creating a voip network... use asterisk?

[Mark Hayward]

I wish to create a voip phone system used by different people accross 
the internet.
I want certain people dotted around the country to be able to connect 
via voip to our main office.
At first this will be using software phones but could extend to hardware 
based phones if it works well.
I would like to run an asterisk server and connect everybody to this 
server from around the country.
This could then connect to the PSTN network via ISDN in the future but 
initially I just want ourselves using internal calls on the voip network.
Would this concept work?
Is there a better way to do this?
Would SIP be the protocol to use to connect remote users to our VOIP 
asterisk server? Would SER be a better alternative?

            Client
               |
Client---------> The Internet--->----Asterisk Server <------Client
               |                                          |
            Client                                PSTN Network


Any help would be most appreiciated.
Thanks