It is possible to make a pc to pc call with Zoiper for desktop if you know the ip address of the other device and the SIP port it's listening to.
For Zoiper Classic the default SIP port is 5060sip:localuser:localpassword@localserver:localserverport/remoteuser(ext)@remoteserver:remoteserverport
for exaple: sip:email@example.com:firstname.lastname@example.org
When dialing with Zoiper3 you do not need to put / in the dialstring.
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:
At the moment this feature is not supported by Zoiper for mobile platforms.