To be able to use Zoiper for incoming and outgoing call, you would need to configure a SIP/IAX account from a VoIP provider. To do that, you can follow some of the instructions below:
Zoiper 3 - Go to Settings -> Preferences -> Accounts -> Create account. Then choose SIP and fill the settings for Domain, Username and Password.
Zoiper for Android - 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.
Zoiper for iOS - Open the Settings menu and go to Accounts, then choose "+" and select whether you have or not an account and how you will configure it (manually or by selecting a provider from the list). If you are going to configure the account manually choose SIP or IAX and fill the information for Domain, Username and Password together with all additional ones, if there are any.
Zoiper for Windows Phone 8 - Run Zoiper and choose "Create" -> "I already have an account" -> "Manual" -> "SIP/IAX" and then fill the settings for the Account name, Domain, Username, Password, etc.