1. Go to Dial-peers -> Companies.
2. Add new one:
Set name = Virt. PBX 1
Set Incoming -> billing pack = Virtual PBX:
Set Their balance -> balance cutoff = yes
Save.
3. Go to Refills menu of Companies which appeared after saving company
4. Add a new one:
Set amount = 10.
Set currency = USD
Set service = Telephony