Most phones won't boot without the battery. Obvious workaround is to connect a power supply (and large cap) directly to the BMS, but in my experience most phones will shut themselves down when they think they've drawn more than the capacity of their battery, even if the voltage is still high. You can sometimes get around that by rebooting the device every couple of hours (and there might be a software/firmware fix in some cases?), but it's a lot less annoying to just accept a small risk of a house fire.