You will be making two calls, one PSTN to PSTN to your SPA-3000 and the second from your SPA-3000 through your Internet connection using VoIP.
The SPA-3000 cannot handle two concurrent PSTN calls on the one phone line service; I suppose it might be possible with a conference call, but that is not likely to work.
A feature of the SPA-3000 is that you can call it remotely and use your VoIP connection to make further a call. Therefore, the SPA-3000 needs to have Internet connectivity.
You cannot initiate the web callback without using a web browser setup with an active Internet connection.
NB: the call from the remote location will not typically be free, there will be a cost from the calling phone to the home phone. The call from the SPA-3000 to a second PSTN line will cost whatever your VSP charges.
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