They are always connected to cell network/tower and internet. Ever noticed how you can do an emergency call without a sim card? That connection potentially works backwards via backdoors as well. Police can access your phone and listen in without you knowing. If they can, so can potentially other unwanted parties. Doing anything crypto related on your phone open you up for attack.
There is also risk of chip/battery failure. Even USB devices need to be reactivated every 1-2 years, otherwise they won't hold enough charge to keep the memory alive, which leads to data loss. You mentioned cold storage, i assume that will be long term, so anything electrial is a no.
For cold storage you want to store your seed words (ideally encrypted) in something
- indestructible - fireproof 1400°C (a house fire is around 1100°C) - waterproof - corrosion proof - shock proof - earthquake proof
