I need some to clear up the confusion between the connection stated in the Title above.
If I understand a little bit :
Site to Site VPN,PPP,HDLC can connect through DSL to internet and communicate with other side.
for the Frame Relay, the FR provider connects you through FR switch on his side , and you will use specific hardware to connect you serial interface to the FR switch.
ISDN has a port that looks like RJ45 but larger, I am not sure where it will connect to, and if the provider will give you DLCI/PVC/CRI just like FR.