I have almost the same problem with our SV8100. Outgoing calls to PSTN have their CID registered in the system. However for incoming calls, CID is not being registered. When using the Caller ID device provided by the telephone company, CID appears so this means that there is indeed an ID. However, the SV8100 doesn't display this ID information in its system.

We also have a GSM gateway device/modem that we use as one of our trunkline connected to the SV8100 to send and receive calls in a cellular phone network. All incoming and outgoing calls on this line have their CID register in the system without any problem.

As for brokeda's comment, may I know on how I can check the loop current?