How do I totally disable my phone? I've bought a very cheap Android 8.1 tablet (which has 2 sim slots) just to play about with.  As it's cheap, the battery isn't wonderful and it has a very short life before it needs recharging.  The main culprit is "Phone Idle" which uses a vast amount of CPU time, whether I have sims installed or not.  I don't actually want to use the phone at all, just the tablet features;  is there any way of completely disabling the phone functions ?
Try turning on airplane mode and turn on wifi.