A free SIP or IAX softelephone client is the only thing that is needed to speak to each other (Exept for the Asterisk server, the Windows PC's, the network, the headphones and the microphones.)
Zoiper does both the SIP and the IAX2 protocol so it's good for testing.
http://www.zoiper.com/The easiest way to connect to the "real world" (pot's) is to order for an sip telephony acount on some external sip telephony wendor. If you do it this way, there will not ne a requirement at all to buy any other hardware.
I am using such a sip based external connection pluss two Grandstream 486 adapters (to use ordinary phones as an alternative to the PC's). Rather cheap and easy to set up and have been running without a bug for approx 3 years now.
X-lite is another sip softphone that used to be quite good. (Personally I prefere the old wersion.)