Hi Vrokad,
Some answers to your questions:
1) if you only want to make internal calls, you don't really need to assign an extension; you can call the person by right-clicking on their name and selecting "call". However, if you really do want to assign an extension, you need to do that in the Line URI field in the user's account in OCS. (click the additional options button on the user page, then under telephony/enterprise voice you'll see the line uri. It should look something liket this:
tel: 1234
(or whatever ext. you want)
Once you've done that, you should be able to call the user from office communicator by dialing 1234
2) you do not need a PBX to do this, OCS will handle the voice and call signalling
3) you can't use other softphones to connect. Just Office Communicator 2007 or one of the third party hard-phones that have been developed by Polycom/Nortel/etc.
Regards,
Matt
