you might want to experiment and observe the behavior of the L5-modem’s firmware in regards to when you initiate a voice-call from inside and outside. you should also inform us (if you don’t think that’s too personal) about how close/far you estimate you are from any tower(s) in your vicinity so we might have a narrower band of speculations going on … every bit helps (that’s how big-daddy-data works anyway 
)
structural materials of the house/construction help us a little too …