my base is entirely underground, for the last 30 min I have been working in a room at y=70 and I started hearing Phantoms, so I went into spectator mode and checked. and found 3 phantoms.  on the wiki it says phantoms wont spawn if the player has "a block above its head which blocks light in any way" this may be related to https://bugs.mojang.com/browse/MC-129760 (water freezing underground)