Do you want use mobile phones as offline/cold storage ? I won't recommended for various reasons.
Most of the comments in this thread criticizing using mobile phone as cold storage is just BS IMHO.
First, mobile phone could get lost or snatched easily
Paper wallets and almost all of the hardware wallets are smaller than most of the smart phones or their size is same as mobile phone. And you are telling me mobile phone could get lost or snatched easily? What about hardware and/or paper wallets? Is there any "protection" for them which makes it harder to get lost or snatched?
Second, mobile phone storage usually only can be used for 5-10 years
What does that suppose to mean?
Third, mobile phone usually is more expensive than paper wallet or hardware wallet such as Trezor or Ledger Wallet
You can get good mobile phones for less price than Trezor's or at a price equal to Ledger Nano's but if you are looking for a cheaper wallet, I recommend Ledger HW1.
I'd recommended paper wallet or hardware wallet than mobile phone for offline/cold storage