• Login to the system administration
• Create an IVR and set '0' = 'Remote Access,' for example
• Create a Trunk to your provider
• Create a DID and point it to the IVR created above

All DID calls will go to the IVR. The caller will have to press '0' (as set above) and supply the system extension number and extension PIN, after which a dial tone will be given and any destination can be dialed.