Yes, you can call anyone over IP (e.g. via public Internet) if the called party owns a VoIP phone and is registered at an SIP registrar, e.g. at example.com (This must be a public server running SIP services).
If the called party does not own a VoIP phone you can still call them via a PSTN gateway that re-routes your call into the PSTN (Plain Old Telephone System). If your provider does not offer this service you will have to set up your own gateway.