There might still be other things in your home which interfere with your cellular signal.
RFI/EMI from badly-shielded electrical systems, transformers, inverters, neon signs, flourescent lights, microwave ovens, electromechanical cash registers, pinball machines, commercial or amateur radio transmitters, industrial machinery, all sorts of stuff.
Everything is supposedly isolated to prevent interference (by technology, by hardware, by design, and by FCC regulations) but the reality is that we live in a noisy world filled with lots of substandard or "aeons old" stuff.
And your building might just have sucky cell reception for other reasons. Concrete and metal. Bad terrain. Low signal bars, fringe service area, not enough towers, etc.
