ANSWER:

An SPA3102 is meant to connect analog phones to a VoIP service, essentially turning the analog phone into a SIP phone. It is not a PSTN gateway. A 7940 is a SIP phone, it requires a connection to a SIP server to work. This is also true with a 3102.

All that advice on the internet assumes you are using a VoIP provider or a PBX as they are required in order to use these devices, unless you are running your own PSTN gateway.

by MaQleod from http://superuser.com/questions/418455