See the discussion here:

http://www.dslreports.com/forum/r26576691-Extend-PSTN-line-over-ethernet-to-a-VOIP-phone

The SPA3102 will bridge a pstn line call with a voip call.  You can configure the PSTN Line Tab of the SPA3102 to pass calls between an attached PSTN line and an IP Phone such as the Cisco/Linksys ip phones where both devices have settings so that you can communicate between the units without registration.  This setting is essential.

There are probably several ways you can configure the ip phone.  For outgoing calls to the PSTN line you can setup a speed dial (or a dial plan) to call the SPA3102 and get a dial tone for the PSTN line.  You would do this by a sip uri that called the SPA3012.  Another way would setup the ip phone with the SPA3102 as an unregistered proxy and then with one-stage dialing dial the pstn number directly.  For this you could setup http digest authentication on the SPA3102.

For incoming calls to the ip phone you would setup the PSTN Line tab's pstn-to-voip gateway to automatically forward any incoming PSTN line call to the ip phone.  You would do this with a dial plan.

I was able to get this working, after combing through many forum postings for config settings required. Attached is the config summary if anyone is interested. I used x-lite 4 for my softphone.

One thing I can't figure out: I have a phone connected to the phone port of the SPA. That phone does not ring for an incoming PSTN call, nor can I use that phone to place a PSTN call. I get dial tone (SPA dialtone) but not PSTN dialtone. I dial numbers but just get a fast busy. Any ideas on how to get that working - for both inbound and outbound calls - would be appreciated.

Daniel

To get the phone attached to the SPA3102 to ring on a PSTN line incoming call you need to have the following settings:

1.  PSTN Answer Delay 16 (number of seconds you want the phone tp ring before the call goes to the pstn-to-voip gateway and is thus transferred to the softphone with your configuration)

2: PSTN Ring Thru Line 1: YES

For outgoing pstn line calls from the phone attached to the SPA3102 you need to setup a Line 1 dial plan to send the call to the PSTN Line.  Something like this: (<#9,:>xx.<:@gw0>|xx.) which if you dial #9 first the call will then be sent to the PSTN Line, otherwise the call will be sent to your voip provider.  On the PSTN Line Tab the voip-to-pstn gateway need to be enabled, and One Stage Dialing should be set to Yes.

Thanks for the additional info. However, removing Cfwd All Dest: gw0 does not get the phone port to ring upon incoming call. Looks like I get a caller ID display on the phone, but no ringing and can't answer the call.

I double checked all the settings (also left Cfwd All Dest: gw0 removed), power cycled the SPA, and it's working. Incoming calls ring through the phone port for 16 seconds, then ring the softphone. Softphone is using port 5060 for outbound and that's working. Will update config doc and repost. Thanks again.