Air gap your devices and don't use them for anything (no email, no texts, chat, etc.... nothing in so there is no malware link possibility) but btc storage. On the rare occasion that you connect (to move coins in or out), use a VPN. You should already be using a VPN full time anyway.
you mean using a phone as a kind of cold storage ? well thats also a cool idea .
using vpn is also a good idea to keep your ip hidden but this does not protect your device on malwares and viruses that can potentially stole your coins , its still advisable to use an anti virus and scan the device before and after you transfer coins .
hardware wallets is a nice lternative next to cold storages and they are less hassel because they are only tiny but they are more powerful and secure .