If you want to safely store your funds in a mobile phone, it's better to use an old one as a cold wallet.
I don't think it is possible to turn a mobile phone in to a true cold wallet. Even on flight mode it still receives and transmits data via a number of channels.
Which ones are you thinking about precisely please? If the sim card is out of order/removed and if there is no connectable wifi network where you are I don't understand how a smartphone could leak a seed outside neither any data.
You can also do the opposite : using an old laptop or tablet that you won't connect till their seed is in use
Cold storage should never connect to the internet, even for a moment. If it does, then it is no longer cold storage.
[/quote]If there is not anymore funds in the seed they store and if you don't plan to use this seed again (like anybody should do) I don't understand what risk there is in using them again. You can use devices as cold wallet without having to sacrifice them, that was my point.