Dynamic IP, PSTN to PSTN, both ways b/w two SPA3102
The easiest way to do what you wish is to go thru a voip provider. This approach helps avoid problems with your routers and problems obtaining and maintaining symbolic ip addresses. The alternative using direct ip calling is more complicated. Your other post seems to indicate that you are encountering voip port blocking so you may be forced to use direct ip calling where you can use any port numbers you wish.
For the simple approach, using voip providers, you setup four voip accounts, subaccounts, or extensions, on the two SPA3102 adapters. On each SPA3102 setup Line 1 Tab and PSTN Line Tab with registrations and then from one SPA3102 you can call the opposite SPA3102's phone by calling the account registered on the Line 1 Tab or receive a dial tone from the opposite SPA3102 by calling the account registered on the PSTN Line tab. It is possible to find voip providers that allow free calling between accounts setup using that provider, or use voip providers that assign and support free calling using iNum numbers.
To use direct ip calling you need to setup and maintain symbolic ip addresses from a Dynamic DNS (DDNS) service, you need to forward the sip ports of the SPA3102 Line 1 and PSTN Line Tabs in your local routers to the SPA3102, and then using direct ip calling you need to call the distant Line 1 Tab to call the phone attached or the distant PSTN Line Tab to receive pstn dial tone from the distant attached pstn line. With direct ip calling it is also possible to setup "one stage dialing" to dial outgoing pstn line calls on the distant SPA3102. To use Direct IP Calling you setup the SPA3102's with Enable IP Dialing, Make and Receive Calls Without Registration, use the symbolic ip addresses that you obtain for each SPA3102, and the sip port numbers you assign to the Line 1 and PSTN Line Tabs.
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