[Phlogiston]

I would not attempt to add an input jack for a cell phone, or other device, without using an isolation transformer, hot chassis or not it's still an Ac/Dc set. Some phones and notepads use metal trim as an antenna, touch the phone, and touch something grounded at the same time, whilst the phone is plugged into an Ac/Dc radio and you could get zapped.
Regards
Arran

Not to mention what could happen if your phone is plugged into it's switching charging supply which is also not isolated.