Here are the some fundamentals of the design of the SPA3102 for what you are trying to achieve.
The incoming PSTN Line call rings the SPA3102. If you have configured "Ring Thru Line 1: Yes" the SPA3102 will ring the phone attached to the SPA3102 for the number of seconds you have configured "PSTN Answer Delay". The incoming call logic will then go to the PSTN To Voip Gateway. To receive incoming pstn line calls from a variety of callers you would normally have "Ring Thru Line 1: Yes" and the default PSTN Answer Delay: 16 (seconds).
If you have configured "Ring Thru Line 1: No" or you have exceeded the configured the PSTN Answer Delay setting, the SPA will send the call to the PSTN to Voip Gateway after the number of seconds configured for "PSTN Answer Delay". Your analog PSTN Line carrier will send the incoming Caller ID during this delay period. In other words the PSTN Answer Delay needs to be long enough to capture the incoming caller id, and if you have "ring thru" long enough for the phone to ring for most other callers. The caller id usually comes in between the first and second ring which can be something like 3 or 4 seconds.
If you have configured "Cfwd Sel1 Caller:" with an incoming caller id the PSTN-to-VoIP gateway will compare the incoming caller id with the caller id you have configured and if it matches will proceed to attempt to forward (or block) the call. If you have configured "Cfwd Sel1 Dest:" with the number you wish to forward (bridge) the call to the PSTN-to-VoIP gateway will use the voip credentials you have configured on the PSTN Line Tab to send the call to the configured destination. The applicable dial plan will be the PSTN Caller Default DP.
At this point calls not matching the configured call forward caller id will go to the PSTN-to-VoIP Gateway and receive a dial tone for dialing. If you wish to block the other callers from receiving this dial tone this dial tone you need the configure "PSTN Caller ID Pattern:" with the caller id of only the caller you have setup under "Cfwd Sel1 Caller" so that all other callers will be blocked from accessing the pstn-to-voip gateway.
Some things to look out for, obviously you need to know the exact incoming caller id of the number you wish to forward. Does it include the country code? Probably not. The forwarded number will probably include the country code if your voip provider requires that.
On the PSTN Line Tab you need to configure your userid/password and other settings to make an outgoing call.
There is a setting "PSTN CID For VoIP CID: Yes/No". This is often called a "spoofed" caller id. Many voip providers will not allow this to be yes as part of their authentication process and will reject the call. The default setting is "No".
If you have configured the same voip account on the Line 1 Tab and the PSTN Line Tab you would normally only set to Register the Line 1 Tab address where you receive incoming voip calls. You would then set the PSTN Line Tab to Register: No, Make Call Without Reg: Yes. Some voip providers require registration to make outgoing calls and if this is applicable in your case you may need a 2d voip account.
