user ---> playSMS master on VPS ---internet---> playSMS slave at home using Raspberry ---> 2 GSM modems
master-slave link is using playnet gateway module
I tried to use OpenVox (not GoIP) for the same topology (now replaced with playSMS slave) but I found that it was not as stable as current solution. The device may not be tuned for internet, maybe it was designed for LAN where the device and server has a fast and more stable connection (unlike internet). You mileage may vary.