The Zoiper Team does not provide any phone numbers.
You should create an account with a VoIP service provider. This works pretty much the same way as when you purchase new phone and you need to input a SIM card in it.
The service provider will give you a SIP or IAX2 account which is essentially your "SIM card". Then you need to set this account into Zoiper to be able to make calls.
If you are not sure which provider to use, try contacting your ISP, they should be able to recommend you one.