Direct Inward Dialling (sometimes referred to as a 'virtual number'). This is a normal phone number on the PSTN that can be called by any phone worldwide, and forwards to your VoIP phone. Many VoIP service providers offer direct inward dialling as part of their service, or as an add-on. There are also some third party companies that offer direct inward dialling, and will forward the call to your VoIP account no matter which service provider you are with.