If you want to make direct calls with Zoiper:
1. you need to have some account to dial with. You can use a real account, or create a dummy account.
2. Dial the remote SIP URI using the following syntax:
sip:remoteuser@remoteserver(IP address of the user):remoteserverport (example: sip:firstname.lastname@example.org:5060)
You should also disable the "Open random port" for SIP in Zoiper's settings. Otherwise Zoiper will listen on random ports above 32000 and you will need to somehow find which is the proper port.