If there is any water inside, it may take a very long time for the phone to dry out completely. Turning it on and "testing it" before it's completely dry is almost certain to "fry" something inside. If the water was sufficiently "contaminated" it may leave residue that shorts something out regardless of how long it's dried.

Some phone makers have had detailed instructions posted on the web, but it's been a long time since I've looked and I don't remember which site(s) I found that had anything. Since we only use $10 throwaway phones it seemed a bunch of trouble for little reward.

My recollection is that the instructions I've seen called for removing the battery immediately but beyond that I wasn't sufficiently impressed with the need (for my kind of phones) to remember much more. My vague recollection is that several days "drying" before testing was suggested.

For a close approximation to the procedure, you might look for treatments for "wet laptops" since similar rules should apply. Since laptops used to be more expensive than 'phones, the makers generally gave more information about how to handle dis-ass-ters of a variety of kinds.

John