What makes skype phone calls work is an extra piece of equipment known as a gateway, which acts as a bridge between the Internet (on one hand) and the Public Switched Telephone Network [PSTN] (on the other). You can think of a gateway as a kind of translator that converts telephone calls in IP-format into traditional signals that ordinary phones can understand (and vice versa). It's also involved in call signalling (the way that incoming calls are signalled to the receiver), so when you dial a landline from a VoIP phone, the gateway converts the call-signalling data into a format that the PSTN can understand (and rings the landline the old-fashioned way).
It then depends on where this gateway is located as to what International Direct Dialing (IDD) code skype uses - so finding out where skypes gateways are would be the only way to find out, assuming skype connects you to the closest one.