I tested the system with my THL W100 (a Chinese dual sim phone) and when connected via wifi, calls came throught the sip instead of the gsm. That's exactly the system's expected behaviour. On the other hand, in my top notch Oneplus 3T the native sip client doesn't work for incoming calls, so they come via gsm.
The system offered by Piranha is a good one and I really hope they will be able to understand what's wrong with my sim.

Thanks gkeeper and DrNewcomb.