Given that you can call each other for free via GV (and presumably SIP, too), why is OBiTALK even important? Assuming that you have a good reason, I'll go on.
Most folks don't have this problem; something is wrong with one or more devices, networks, or accounts.
Try calling between your respective PCs with OBiAPP. If that's fine, call between device and app to see which device is causing trouble. If it's not fine, test at work/school/friend's to see which network is problematic.
On a bad call, what does Call Status show (in both OBi devices)? If you immediately call back, goes it get good agian for another 15 minutes? Please post some details: what countries you are in, ISPs, modems, routers, etc.