To be able to use Zoiper, you would need to have SIP or IAX2 account. To setup Zoiper for using in home VoIP network, you should have a PBX where to create the accounts (or some accounts from a VoIP provider) and then to configure these into Zoiper by following some of the next instructions. If everything is setup correctly, you should be able to make calls between the accounts.
To configure an account in Zoiper:
Go to Configuration -> Accounts -> Add account -> SIP/IAX -> Select yes if you already have an account -> Manual configuration and fill the required settings for Host, Username and Password. If any additional settings (STUN, Outbound proxy, etc.) are required, fill them as well.