The disguising of a military weapons in the form of common civilian used equipment to trick your opponent is a war crime.
It was a war crime in 2008 when a bomb was disguised as a spare tire in an SUV used to kill the head of Hezbollah’s international operations, whether we agree the target needed to be taken out or not. A drone strike would be “lawful” a car bomb is not.
A cell phone is common civilian equipment. This isn’t “whatever I think.”
“stress that booby-traps associated with objects in normal civilian daily use are prohibited, and that booby-traps must not be used in association with protected persons, protected objects (such as medical supplies, gravesites and cultural or religious property) or internationally recognized protective emblems or signs (such as the red cross and red crescent).[3] Several manuals further specify that booby-traps must not be used in connection with certain objects likely to attract civilians, such as children’s toys.”
A cell phone is a normal civil daily use item and would attract use by civilians.
This specifically would come from Rule 80, pertaining to booby traps. https://ihl-databases.icrc.org/en/customary-ihl/v1/rule80