SoftSip to SoftSip over same Lan segment. User Busy message

Have installed Asterisk server - have 2 PC’s running on same Lan segment using BOL SipPhones. Can receive voice prompts etc from server from each SipPhone however when I try to connect to each sipphone from the other I get “The user at extension (at the extension number) is on the phone”.
I have obviously missed some config setting on the Asterisk server - currently I am only trying to get a Lan sipphone system running.

Regards

David

You’ll need to post the relevant bits of your sip.conf and extensions.conf - but only the relevant bits at this stage!