How to discharge a dead phone?
Be careful with that advice you have been given, as you will write off the battery from a safety perspective if you drain it down to a certain threshold. If you want to dispose of the battery you may wish to drain it - carefully. However in your instance, you more likely want to establish whether the phone is the issue or the battery. I have had a phone before that shorts out batteries so we can't always assume the battery is to blame. Unfortunately in your instance you have what I would call a built in battery so you have to decide whether you want to try swapping out the battery or replacing/repairing the phone.