Voip solution ---I just looking for a VoIP solution advice

I need to make Voice over IP calls between 2 countries, to landlines and mobile numbers: In both countries I ALREADY have "landline" service (PSTN) with unlimited call plans. I would prefer NOT To use an Asterix, Elastix or similar solution, in that case I rather to use a low cost solution such as the old Linksys SPA3102 or similar device (with FXS, FXO ports) I know that there are “virtual PBX” services with low costs BUT if I already pay for unlimited calls service, and I DO NOT want to pay additional monthly costs or additional costs per call. So, What What I need ? : I need an explicit diagram of a low cost solution using my current PSTN lines . Preferably be based on ATA-type devices like the Linksys SPA 3102 or something similar cost solutions, probably with VPN links. I ONLY ask for THE DIAGRAM WITH PART names and models similar as my diagram.